Current handling of WriteTimeout for http2 does not
extend the timeout on new streams. Disable the WriteTimeout
in http2 for 1.8 release.

Fixes test added in https://golang.org/cl/34723

Updates golang/go#18437

Change-Id: I366899fb4ff2e740610cad71e004141d092699a2
Reviewed-on: https://go-review.googlesource.com/34724Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>