The existing implementation passes hostname:port to the ServerName
field of the client's TLS config. This is passed to the server
incorrectly as the ServerName in the client hello. This change adds a
function to strip the port from the host when passing it to the TLS
config.

Change-Id: I03714ffc7f21d87c375f8f07392ef02bbe76da66
Reviewed-on: https://go-review.googlesource.com/34728
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>