[release-branch.go1.11] net/http: update bundled x/net/http2

This updates x/net/http2 to x/net on the latest commit of x/net on
release-branch.go1.11 branch, golang/net@a4630153038d3cb8c57f, for:

	[release-branch.go1.11] http2: don't leak streams on broken body
	https://golang.org/cl/154237

Fixes #28673

Change-Id: I8f40c69502fa95d1d9dff1ab68444640444442aa
Reviewed-on: https://go-review.googlesource.com/c/154298


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>