Updates http2 to x/net git rev 520af5de654d for
https://golang.org/cl/18370

Fixes #13659

Change-Id: I920eaff6036ac22c500a97449826c6b12f873d7f
Reviewed-on: https://go-review.googlesource.com/18371
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>