net/http: don't send chunked encoding on 204 responses

net/http: don't send chunked encoding on 204 responses

RFC 2616: "The 204 response MUST NOT include a message-body,
and thus is always terminated by the first empty line after
the header fields."

Previously we'd trigger chunked encoding by default on
responses, and then when finishing the request we'd write the
chunk trailers, which counted as a message-body.

Fixes #4454

R=golang-dev
CC=golang-dev
https://golang.org/cl/6782139