Only set the X-Forwarded-Proto for SSL vhosts

Only set the X-Forwarded-Proto header on the SSL terminating (ie. public
facing) vhost. If we set it again on the backend app vhost we clobber
the header coming from the SSL terminating vhost.
Merge pull request #407 from alphagov/only-forward-ssl-proto

Only set the X-Forwarded-Proto for SSL vhosts