Updates x/net/http2 to git rev b2ed34f for https://golang.org/cl/18727

Updates #14029 (fixes it enough for Go 1.6)
Fixes #13961

Change-Id: Id301247545507671f4e79df0e7c6ec9c421d5a7c
Reviewed-on: https://go-review.googlesource.com/18728
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>