http: allow handlers to send non-chunked responses

http: allow handlers to send non-chunked responses

Currently all http handlers reply to HTTP/1.1 requests with
chunked responses.  This patch allows handlers to opt-out of
that behavior by pre-declaring their Content-Length (which is
then enforced) and unsetting their Transfer-Encoding or
setting it to the "identity" encoding.

R=rsc, bradfitzwork
CC=golang-dev
https://golang.org/cl/4245058