Support text/csv responses

[antoneladestito]

Hello! I am using openapi-python-client and it seems that text/csv is unsupported:

Warning(s) encountered while generating. Client was generated, but some pieces may be missing

Unsupported content_type {'text/csv':

[dbanty]

#780 should fix this once finished, though data will come through as plain text, not a CSV type

[dbanty]

This now works (as plain text) in 0.16.0