text: Title: Issue with @api.representation('application/xml')
  Body: I was following the example in help doc to create the below decorator.
  ```python
  @api.representation('application/xml')
  def xml(data, code, headers=None):
      resp = make_response(dicttoxml.dicttoxml(data), code)   
      resp.headers.extend(headers or {})
      return resp
  ```
  And I also set up
  ```
  api = Api(app, default_mediatype='application/json' )
  ```
  The problem is when I launched swagger dev portal and clicked link: http://localhost:5000/swagger.json ,it returned xml format rather than json format, which doesn't make sense. 
  