Previously, we didn't handle absolute DNS names in certificates the same
way as Chromium, and we probably shouldn't diverge from major browsers.

Change-Id: I56a3962ad1002f68b5dbd65ae90991b82c2f5629
Reviewed-on: https://go-review.googlesource.com/5692Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>