Enable support for the content-type 'application/json' in the Error Responses

Feature to customize the Content-Type to application/json for all the Error Messages returned by the APIcast, including:

• AUTHENTICATION FAILED ERROR.
• AUTHENTICATION MISSING ERROR.
• NO MATCH ERROR.
• Traffic Limits Reached (recently implemented by default as 429).

And format the response according to this Content-Type as well.

A suggestion of implementing this feature could be based upon the Accept header of the HTTP Request being received, however changes on the UI might also address it.