SSL detection correction

$protocol always equals 'https' because $_SERVER['HTTPS'] !== 'off' always evaluates to true which causes the shared links to always use https even if the server doesn't use ssl.

This line was tested on two different servers running php5-fpm 5.3.10 with nginx on ubuntu 12.04, one configured with ssl and the other without ssl.
With the original test, $protocol always equals 'https', with the modification it correctly equals 'https' on the server with ssl and 'http' on the other server.