This CL also updates the bundled http2 package with the h2 fix from
https://golang.org/cl/17757

Fixes #13159

Change-Id: If0e3b4bd04d0dceed67d1b416ed838c9f1961576
Reviewed-on: https://go-review.googlesource.com/17758Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>