Updates x/net/http2 to git rev 493a262 for https://golang.org/cl/19223

Fixes #14227

Change-Id: I626122811138fb3d88e4eea83f8da3fdcf91e0dc
Reviewed-on: https://go-review.googlesource.com/19250Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>