Can't ask the requests client to follow redirects

[abompard]

According to the documentation, it should be possible to ask Bravado to follow HTTP redirects using the _request_options parameter. Unfortunately, RequestConfig doesn't know about it and follow_redirect ends up in RequestConfig.additional_parameters which isn't used anywhere.
And then, RequestsClient.separate_params will try to find the follow_redirects option inside the requests params, which will never be there because requests_params only contains the parameters included in the Swagger spec, and will thus set it to False.

I think the detection of follow_redirects should be removed from RequestsClient.separate_params and added to RequestConfig instead.