As rsc suggested after change 58a6bdac3d12 was committed, we
now read the first byte of Request.Body when the
Request.ContentLength is 0 to disambiguate between a truly
zero-length body and a body of unknown length where the user
didn't set the ContentLength field.

This was also causing the reverse proxy problem where incoming
requests (which always have a body, of private type http.body,
even for 0-lengthed requests) were being relayed to the http
Transport for fetching, which was serializing the request as a
chunked request (since ContentLength was 0 and Body was
non-nil)

Fixes #1999

R=golang-dev, kevlar
CC=golang-dev
https://golang.org/cl/4628063