Compliance issue addressed here: POST requests carrying form data are required
to use "identity" transfer encoding by common nginx and apache server configurations,
e.g. wordpress.com (and many others). So, Request needed to be able to send
non-chunked encodings.

Thus, Request is extended to support identity and chunked encodings, like
Response.  Since the Read() and Write() logic are shared by both (and are
quite long), it is exported in a separate file transfer.go.

R=rsc
CC=golang-dev
https://golang.org/cl/217048