Codec issues.

[techdragon]

There appears to be an issue when the library tries to handle a blank API response in an application/json based API when the response is "whitespace only", that is to say its empty, but not really empty, and the library appears to be trying to fall back and work out if it can parse it as XML when that has not been specified as a valid MIME type. The Swagger API Spec for this application says it consumes and produces application/json on all of the endpoints, yet the library is trying to get a text/xml codec?

Traceback (most recent call last):
...
  File "/venv/lib/python3.6/site-packages/pyswagger/contrib/client/requests.py", line 74, in request
    raw=six.BytesIO(rs.content).getvalue()
  File "/venv/lib/python3.6/site-packages/pyswagger/io.py", line 416, in apply_with
    data = self.__op._mime_codec.unmarshal(content_type, self.raw, _type=_type, _format=_format, name=name)
  File "/venv/lib/python3.6/site-packages/pyswagger/primitives/codec.py", line 34, in unmarshal
    raise Exception('Could not find codec for %s, data: %s, args: %s' % (mime, data, kwargs))
Exception: Could not find codec for text/xml, data: b' ', args: {'_type': 'object', '_format': None, 'name': 'ErrorResponse'}