[Multiple issue asignees] Fix json schema for issues

spec/fixtures/api/schemas/issue.json

@@ -40,7 +40,9 @@
         "additionalProperties": false
       }
     },
-    "assignee": {
+    "assignees": {
+      "type": "array",
+      "items": {
         "type": ["object", "null"],
         "required": [
           "id",
@@ -53,8 +55,8 @@
           "name": { "type": "string" },
           "username": { "type": "string" },
           "avatar_url": { "type": "uri" }
-      },
-      "additionalProperties": false
+        }
+      }
     },
     "subscribed": { "type": ["boolean", "null"] }
   },