FIX Director::forceSSL and forceWWW not setting Vary header

If you have a Varnish box in front of a SilverStripe install, and
you call forceSSL, the Vary header wouldnt get sent. As a result
Varnish would respond with the same redirect reponse after the
redirect, leading to an infinite loop
Merge pull request #2200 from hafriedlander/fix/varies

FIX Director::forceSSL and forceWWW not setting Vary header