[Multiple issue asignees] Fix json schema for issues

spec/fixtures/api/schemas/issue.json

@@ -40,21 +40,23 @@
         "additionalProperties": false
       }
     },
-    "assignee": {
-      "type": ["object", "null"],
-      "required": [
-        "id",
-        "name",
-        "username",
-        "avatar_url"
-      ],
-      "properties": {
-        "id": { "type": "integer" },
-        "name": { "type": "string" },
-        "username": { "type": "string" },
-        "avatar_url": { "type": "uri" }
-      },
-      "additionalProperties": false
+    "assignees": {
+      "type": "array",
+      "items": {
+        "type": ["object", "null"],
+        "required": [
+          "id",
+          "name",
+          "username",
+          "avatar_url"
+        ],
+        "properties": {
+          "id": { "type": "integer" },
+          "name": { "type": "string" },
+          "username": { "type": "string" },
+          "avatar_url": { "type": "uri" }
+        }
+      }
     },
     "subscribed": { "type": ["boolean", "null"] }
   },