Also, don't read from the Request.Headers in the http Server code once
ServeHTTP has started. This is partially redundant with documenting
that handlers shouldn't mutate request, but: the space is free due to
bool packing, it's faster to do the checks once instead of N times in
writeChunk, and it's a little nicer to code which previously didn't
play by the unwritten rules. But I'm not going to fix all the cases.

Fixes #14940

Change-Id: I612a8826b41c8682b59515081c590c512ee6949e
Reviewed-on: https://go-review.googlesource.com/21530
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>