Attach original Request object to Response object.

Allows for quickly creating Requests similar to those already made, or for
examining Request object manipulation by third party hooks or authentication
methods.

This is particularly useful when using requests with something like oauthlib: https://github.com/requests/requests-oauthlib

There appeared to be an instance of the original request object passed along as far as `Session.send()`, but it was never used after that. So, I grabbed it and attached it to the Response object.