Introduce IClientRequest to avoid publically exposing all the ugly, and supposed to be private, moving parts of _newclient.Request via Response.request.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/redirectagent-final-uri-5435@38171 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb