Updates x/net/http2 to git rev 31df19d6 for changes since Go 1.6.

The main change was https://go-review.googlesource.com/19726 (move
merging of HEADERS and CONTINUATION into Framer), but there were a few
garbage reduction changes too.

Change-Id: I882443d20749f8638f637a2835efe92538c95d31
Reviewed-on: https://go-review.googlesource.com/21365
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>