Updates to x/net git revision 9946ad7

Change-Id: I95c03daf382667002a5b22f184bd9b7d18144913
Reviewed-on: https://go-review.googlesource.com/16066Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>