tls_wrap: use localhost if options.host is empty

tls.connect(options) with no options.host should accept a certificate
with CN: 'localhost'. Fix Error: Hostname/IP doesn't match
certificate's altnames: "Host: undefined. is not cert's CN: localhost"

'localhost' is not added directly to defaults because that is not
always desired (for example, when using options.socket)

PR-URL: https://github.com/iojs/io.js/pull/1493
Fixes: https://github.com/iojs/io.js/issues/1489


Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>
Reviewed-By: Roman Reiss <me@silverwind.io>