net/http: update x/net/http2

Updated bundle of golang.org/x/net/http2 to rev c95266

Change-Id: Id6df66a8f4eadd6a100f359fd6cac981653dafa6
Reviewed-on: https://go-review.googlesource.com/16448Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>