Some headers, which are set or modified by the http library,
are not written to the standard http.Request.Header and are
not included as part of http.Response.Request.Header.

Exposing all headers alleviates this problem.

This is not a complete solution to 19761 since it does not have http/2 support.

Updates #19761

Change-Id: Ie8d4f702f4f671666b120b332378644f094e288b
Reviewed-on: https://go-review.googlesource.com/67430
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>