ENH: ensure we use custom adapters if mounted.

This allows responses to be used with custom adapters.

Tested on 2.7 with requests 2.4.1
I meant that because send is mocked, self in _on_request refers to the responses class, not the original Session one, and the only public way to access adapters is from the session AFAIK (I don't know anything about requests internals either, I just quickly looked at it)