This change excludes internal UDP header size from a result of number of
bytes written on WriteTo.

Change-Id: I847d57f7f195657b6f14efdf1b4cfab13d4490dd
Reviewed-on: https://go-review.googlesource.com/9196Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>