I wasn't emulating the stateful file-object in my response stub,
so urllib3 wasn't decompressing gzipped bodies properly. This
should fix that problem.
Thanks @bryanhelmig for the motivation to dig into this.
This commit changes the whole core internal flow of requests.
Now, requests are actually physically made lazily when a response
is requested. This allows the entire request to be sent at once.
Otherwise, it would be impossible to compare whether requests have
already been recorded, since httplib.send() allows you to effectively
stream requests over HTTP.
