autocert: validate SNI values more, add tests Change-Id: I810c8dcc90c056d7fa66bba59c0936f54aabdfc7 Reviewed-on: https://go-review.googlesource.com/42497 Run-TryBot: Alex Vaghin <ddos@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Alex Vaghin <ddos@google.com>