Fix $_SERVER['SERVER_PORT'] handling

While installing Framadate on Apache, but proxyfied by an Nginx,
$_SERVER['SERVER_PORT'] failed to be correctly handled.

$_SERVER['SERVER_PORT'] is then a string and not an integer, so it was
added in the response of get_server_name() function, breaking all the
css/js links.

I don't know if there is other situations where the bug appears.

Schema of the installation:

Nginx (443) -> Apache2 (80) + Framadate

Please note that using port 80 on Nginx worked.