Give the response some context, just like requests does.

See https://github.com/kennethreitz/requests/blob/master/requests/adapters.py#L180-L189.

I'm using httmock to test a new API wrapper and checking the internals of the `request` object to ensure that my wrapper is setting up the calls correctly. In https://github.com/kennethreitz/requests/blob/master/requests/adapters.py#L180-L189 , `requests` adds both the `request` and `request.url` as attributes on the response. This does the same.