Add a _compress_response option for operation calls

[AlstonLin]

When Yelp/bravado-core#231 is merged to bravado-core, bravado should have an easy way to initiate requests for compressed responses.
Specifically, users should be able to add a _compress_response=True flag, which would then cause an Accept-Encoding: deflate header to be automatically included in the request.

[AlstonLin]

Closing this as it seems the requests module already sends this header by default, and also handles it automatically