MUST use problem json as default response

The content type for the default response MUST be application/problem+json.

Invalid Example

The default response in this example incorrectly uses application/json as the content type.

responses:
   ...
get:
  summary: Get User Info by User ID
   tags: []
   responses:
     ...
     default:
       description: ...
       content:
         application/json:
           schema:
             $ref: ../models/Problem.yaml

Valid Example

The default response in this example correctly uses application/problem+json as the content type.

responses:
   ...
get:
  summary: Get User Info by User ID
   tags: []
   responses:
     ...
     default:
       description: ...
       content:
         application/problem+json:
           schema:
             $ref: ../models/Problem.yaml

Zalando Guideline 151