Fix JSON schema public_api/v4/merge_request.json

Prior this commit this schema expected an optional property named
`properties`.

This commit:
* remove nested `properties`
* references milestone schema for property `milestone`
* references user basic schema for property `assignees`

spec/fixtures/api/schemas/public_api/v4/merge_request.json

 {
   "type": "object",
   "properties" : {
-    "properties" : {
-      "id": { "type": "integer" },
-      "iid": { "type": "integer" },
-      "project_id": { "type": "integer" },
-      "title": { "type": "string" },
-      "description": { "type": ["string", "null"] },
-      "state": { "type": "string" },
-      "merged_by": {
-        "type": ["object", "null"],
-        "properties": {
-          "name": { "type": "string" },
-          "username": { "type": "string" },
-          "id": { "type": "integer" },
-          "state": { "type": "string" },
-          "avatar_url": { "type": "uri" },
-          "web_url": { "type": "uri" }
-        },
-        "additionalProperties": false
+    "id": { "type": "integer" },
+    "iid": { "type": "integer" },
+    "project_id": { "type": "integer" },
+    "title": { "type": "string" },
+    "description": { "type": ["string", "null"] },
+    "state": { "type": "string" },
+    "merged_by": {
+      "type": ["object", "null"],
+      "properties": {
+        "name": { "type": "string" },
+        "username": { "type": "string" },
+        "id": { "type": "integer" },
+        "state": { "type": "string" },
+        "avatar_url": { "type": "uri" },
+        "web_url": { "type": "uri" }
       },
-      "merge_user": {
-        "type": ["object", "null"],
-        "properties": {
-          "name": { "type": "string" },
-          "username": { "type": "string" },
-          "id": { "type": "integer" },
-          "state": { "type": "string" },
-          "avatar_url": { "type": "uri" },
-          "web_url": { "type": "uri" }
-        },
-        "additionalProperties": false
-      },
-      "merged_at": { "type": ["string", "null"] },
-      "closed_by": {
-        "type": ["object", "null"],
-        "properties": {
-          "name": { "type": "string" },
-          "username": { "type": "string" },
-          "id": { "type": "integer" },
-          "state": { "type": "string" },
-          "avatar_url": { "type": "uri" },
-          "web_url": { "type": "uri" }
-        },
-        "additionalProperties": false
-      },
-      "closed_at": { "type": ["string", "null"], "format": "date-time" },
-      "created_at": { "type": "string", "format": "date-time" },
-      "updated_at": { "type": "string", "format": "date-time" },
-      "target_branch": { "type": "string" },
-      "source_branch": { "type": "string" },
-      "upvotes": { "type": "integer" },
-      "downvotes": { "type": "integer" },
-      "author": {
-        "type": "object",
-        "properties": {
-          "name": { "type": "string" },
-          "username": { "type": "string" },
-          "id": { "type": "integer" },
-          "state": { "type": "string" },
-          "avatar_url": { "type": "uri" },
-          "web_url": { "type": "uri" }
-        },
-        "additionalProperties": false
-      },
-      "assignee": {
-        "type": ["object", "null"],
-        "properties": {
-          "name": { "type": "string" },
-          "username": { "type": "string" },
-          "id": { "type": "integer" },
-          "state": { "type": "string" },
-          "avatar_url": { "type": "uri" },
-          "web_url": { "type": "uri" }
-        },
-        "additionalProperties": false
-      },
-      "assignees": {
-        "items": {
-          "$ref": "./merge_request.json"
-        }
+      "additionalProperties": false
+    },
+    "merge_user": {
+      "type": ["object", "null"],
+      "properties": {
+        "name": { "type": "string" },
+        "username": { "type": "string" },
+        "id": { "type": "integer" },
+        "state": { "type": "string" },
+        "avatar_url": { "type": "uri" },
+        "web_url": { "type": "uri" }
       },
-      "source_project_id": { "type": "integer" },
-      "target_project_id": { "type": "integer" },
-      "labels": {
-        "type": "array",
-        "items": {
-          "type": "string"
-        }
+      "additionalProperties": false
+    },
+    "merged_at": { "type": ["string", "null"] },
+    "closed_by": {
+      "type": ["object", "null"],
+      "properties": {
+        "name": { "type": "string" },
+        "username": { "type": "string" },
+        "id": { "type": "integer" },
+        "state": { "type": "string" },
+        "avatar_url": { "type": "uri" },
+        "web_url": { "type": "uri" }
       },
-      "work_in_progress": { "type": "boolean" },
-      "milestone": {
-        "type": ["object", "null"],
-        "properties": {
-          "id": { "type": "integer" },
-          "iid": { "type": "integer" },
-          "project_id": { "type": ["integer", "null"] },
-          "group_id": { "type": ["integer", "null"] },
-          "title": { "type": "string" },
-          "description": { "type": ["string", "null"] },
-          "state": { "type": "string" },
-          "created_at": { "type": "string", "format": "date-time" },
-          "updated_at": { "type": "string", "format": "date-time" },
-          "due_date": { "type": "string", "format": "date-time" },
-          "start_date": { "type": "string", "format": "date-time" }
-        },
-        "additionalProperties": false
+      "additionalProperties": false
+    },
+    "closed_at": { "type": ["string", "null"], "format": "date-time" },
+    "created_at": { "type": "string", "format": "date-time" },
+    "updated_at": { "type": "string", "format": "date-time" },
+    "target_branch": { "type": "string" },
+    "source_branch": { "type": "string" },
+    "upvotes": { "type": "integer" },
+    "downvotes": { "type": "integer" },
+    "author": {
+      "type": "object",
+      "properties": {
+        "name": { "type": "string" },
+        "username": { "type": "string" },
+        "id": { "type": "integer" },
+        "state": { "type": "string" },
+        "avatar_url": { "type": "uri" },
+        "web_url": { "type": "uri" }
       },
-      "merge_when_pipeline_succeeds": { "type": "boolean" },
-      "merge_status": { "type": "string" },
-      "sha": { "type": "string" },
-      "merge_commit_sha": { "type": ["string", "null"] },
-      "user_notes_count": { "type": "integer" },
-      "changes_count": { "type": "string" },
-      "should_remove_source_branch": { "type": ["boolean", "null"] },
-      "force_remove_source_branch": { "type": ["boolean", "null"] },
-      "discussion_locked": { "type": ["boolean", "null"] },
-      "web_url": { "type": "uri" },
-      "squash": { "type": "boolean" },
-      "time_stats": {
-        "time_estimate": { "type": "integer" },
-        "total_time_spent": { "type": "integer" },
-        "human_time_estimate": { "type": ["string", "null"] },
-        "human_total_time_spent": { "type": ["string", "null"] }
+      "additionalProperties": false
+    },
+    "assignee": {
+      "type": ["object", "null"],
+      "properties": {
+        "name": { "type": "string" },
+        "username": { "type": "string" },
+        "id": { "type": "integer" },
+        "state": { "type": "string" },
+        "avatar_url": { "type": "uri" },
+        "web_url": { "type": "uri" }
       },
-      "allow_collaboration": { "type": ["boolean", "null"] },
-      "allow_maintainer_to_push": { "type": ["boolean", "null"] },
-      "references": {
-        "short": {"type": "string"},
-        "relative": {"type": "string"},
-        "full": {"type": "string"}
+      "additionalProperties": false
+    },
+    "assignees": {
+      "type": "array",
+      "items": {
+        "$ref": "user/basic.json"
+      }
+    },
+    "source_project_id": { "type": "integer" },
+    "target_project_id": { "type": "integer" },
+    "labels": {
+      "type": "array",
+      "items": {
+        "type": "string"
       }
     },
-    "required": [
-      "id", "iid", "project_id", "title", "description",
-      "state", "created_at", "updated_at", "target_branch",
-      "source_branch", "upvotes", "downvotes", "author",
-      "assignee", "source_project_id", "target_project_id",
-      "labels", "work_in_progress", "milestone", "merge_when_pipeline_succeeds",
-      "merge_status", "sha", "merge_commit_sha", "user_notes_count",
-      "should_remove_source_branch", "force_remove_source_branch",
-      "web_url", "squash"
-    ],
-    "head_pipeline": {
+    "work_in_progress": { "type": "boolean" },
+    "milestone": {
       "oneOf": [
         { "type": "null" },
-        { "$ref": "pipeline/detail.json" }
+        { "$ref": "milestone.json" }
       ]
+    },
+    "merge_when_pipeline_succeeds": { "type": "boolean" },
+    "merge_status": { "type": "string" },
+    "sha": { "type": "string" },
+    "merge_commit_sha": { "type": ["string", "null"] },
+    "user_notes_count": { "type": "integer" },
+    "changes_count": { "type": "string" },
+    "should_remove_source_branch": { "type": ["boolean", "null"] },
+    "force_remove_source_branch": { "type": ["boolean", "null"] },
+    "discussion_locked": { "type": ["boolean", "null"] },
+    "web_url": { "type": "uri" },
+    "squash": { "type": "boolean" },
+    "time_stats": {
+      "time_estimate": { "type": "integer" },
+      "total_time_spent": { "type": "integer" },
+      "human_time_estimate": { "type": ["string", "null"] },
+      "human_total_time_spent": { "type": ["string", "null"] }
+    },
+    "allow_collaboration": { "type": ["boolean", "null"] },
+    "allow_maintainer_to_push": { "type": ["boolean", "null"] },
+    "references": {
+      "short": {"type": "string"},
+      "relative": {"type": "string"},
+      "full": {"type": "string"}
     }
+  },
+  "required": [
+    "id", "iid", "project_id", "title", "description",
+    "state", "created_at", "updated_at", "target_branch",
+    "source_branch", "upvotes", "downvotes", "author",
+    "assignee", "source_project_id", "target_project_id",
+    "labels", "work_in_progress", "milestone", "merge_when_pipeline_succeeds",
+    "merge_status", "sha", "merge_commit_sha", "user_notes_count",
+    "should_remove_source_branch", "force_remove_source_branch",
+    "web_url", "squash"
+  ],
+  "head_pipeline": {
+    "oneOf": [
+      { "type": "null" },
+      { "$ref": "pipeline/detail.json" }
+    ]
   }
 }