The default is 10MB, like http2, but can be configured with a new
field http.Transport.MaxResponseHeaderBytes.

Fixes #9115

Change-Id: I01808ac631ce4794ef2b0dfc391ed51cf951ceb1
Reviewed-on: https://go-review.googlesource.com/21329
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>