Updates x/net/http2 to git rev 96dbb961 for golang.org/cl/23002

Fixes #15366
Updates #15134 (server part remains)

Change-Id: I29336e624706f906b754da66381a620ae3293c6c
Reviewed-on: https://go-review.googlesource.com/23003Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>