Updates x/net/http2 to 3b99394 for golang.org/cl/23205

And associated tests.

Fixes #12580

Change-Id: I1f4b59267b453d241f2afaa315b7fe10d477e52d
Reviewed-on: https://go-review.googlesource.com/23206Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>