This is a backwards-incompatible change that will store headers
as a list rather than a dictionary. The reason being that you can
have multiple values for a single header, and concatenating them
together with commas can create an unparseable string (sometimes
the header values can have commas in them)
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.
