[python] Binary string response encoding with Python 3

When using binary string responses under Python 3, the encoding defaults to UTF8 which is incompatible with binary data. This looks to stem from Python 3 having str default to UTF8 and a new type called bytes which represents binary data. This is a significant departure from functionality in Python 2 and you can read more about it.

This issue does not exist in Python 2.

Swagger Revision: 18262c1b6132a28438e2c3d9426458c413f011f8 (master as of right now)

Response schema:

schema:
  type: string
  format: binary

Example Error:

Traceback (most recent call last):
  File "./dr.py", line 13, in <module>
    "document_type": "pdf",                                         # pdf or xls or xlsx
  File "/usr/local/lib/python3.5/site-packages/docraptor/apis/doc_api.py", line 203, in create_doc
    callback=params.get('callback'))
  File "/usr/local/lib/python3.5/site-packages/docraptor/api_client.py", line 322, in call_api
    response_type, auth_settings, callback)
  File "/usr/local/lib/python3.5/site-packages/docraptor/api_client.py", line 149, in __call_api
    post_params=post_params, body=body)
  File "/usr/local/lib/python3.5/site-packages/docraptor/api_client.py", line 358, in request
    body=body)
  File "/usr/local/lib/python3.5/site-packages/docraptor/rest.py", line 208, in POST
    body=body)
  File "/usr/local/lib/python3.5/site-packages/docraptor/rest.py", line 171, in request
    r.data = r.data.decode('utf8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe2 in position 10: invalid continuation byte

I'm not sure what a good general fix would look like but I have a work-around for our use by delaying byte decoding until right before JSON deserialization and short-circuiting str. You can see my fix directly on generated code.

Related Issues and PRs:

• https://github.com/DocRaptor/docraptor-python/issues/2
• https://github.com/DocRaptor/docraptor-python/pull/3