Updates http2 to x/net/http2 git rev d4223d6710a for:

    http2: refund connection flow control on DATA frames received after reset
    https://golang.org/cl/46591

Fixes #46591

Change-Id: I5e6999599e921fed279c6d29ae77439191e99615
Reviewed-on: https://go-review.googlesource.com/47096
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>