Adds support for `requests`.

Requests wasn't working because it uses urllib3, which uses a custom version of HTTPConnection. I introduced support by mocking and patching this class. Also added tweaks to maintain compatibility with some weird things urllib3 does.

Closes issue #7.
I needed to add additional fields to the mocked response object to make it work with requests/urllib3. These are just corresponding updates to the responses.yaml test fixture to make those tests work.