crypto/tls: always send a Certificate message if one was requested.
crypto/tls: always send a Certificate message if one was requested.

If a CertificateRequest is received we have to reply with a
Certificate message, even if we don't have a certificate to offer.

Fixes issue 3339.

R=golang-dev, r, ality
CC=golang-dev
http://codereview.appspot.com/5845067
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go