Don't let transports rewrite the request.

Now that we're preparing Content-Length and Transfer-Encoding
before passing a request to the HTTP engine, we have no strict
need to rewrite requests in the transport.

The nice upside of this change is that the transport becomes
even less obvious when it's in place.
Merge pull request #390 from square/jwilson_0101_prepare_request

Don't let transports rewrite the request.