Updates http2 to x/net/http2 git rev 3d7ac2a5d for:

    http2: fix Server race
    https://golang.org/cl/20704

Fixes #20704

Change-Id: I803288f94a8a462ad0960b0c29c2dfee27b9f1ae
Reviewed-on: https://go-review.googlesource.com/46093
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Tom Bergan <tombergan@google.com>
Reviewed-by: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>