Add required fields to the 'assignee' property on the Issue JSON schema

spec/fixtures/api/schemas/issue.json

@@ -40,11 +40,21 @@
       }
     },
     "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
+    },
     "subscribed": { "type": ["boolean", "null"] }
   },
   "additionalProperties": false