vendor: update golang.org/x/net/http2/hpack
Updates golang.org/x/net/http2/hpack to x/net git rev
4dbf598 for:
http2/hpack: remove unused pair function from package
https://golang.org/cl/43851
http2/hpack: remove pair function and initialize directly
https://golang.org/cl/43190
http2/hpack: move initialization to a static table
https://golang.org/cl/43090
Updates #6853
Change-Id: I933a094623143c7333a0da7867d5d2e41da05234
Reviewed-on: https://go-review.googlesource.com/44007
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>