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
But on_request is being called identically unless there's magic I'm missing. It literally binds self to self._on_request, so when it's called it'd be called with the additional arguments.

Does it simply not work if you  pass ._on_request instead of callback?

I'm just confused why this would change behavior since AFAIK this is identical from Python's POV