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.

app/classes/Framadate/Utils.php

@@ -26,7 +26,7 @@ class Utils {
      */
     public static function get_server_name() {
         $scheme = ((isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] === 'on') || (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https')) ? 'https' : 'http';
-        $port = in_array($_SERVER['SERVER_PORT'], [80, 443], true) ? '' : ':' . $_SERVER['SERVER_PORT'];
+        $port = in_array($_SERVER['SERVER_PORT'], ['80', '443'], true) ? '' : ':' . $_SERVER['SERVER_PORT'];
         $dirname = dirname($_SERVER['SCRIPT_NAME']);
         $dirname = $dirname === '\\' ? '/' : $dirname . '/';
         $dirname = str_replace('/admin', '', $dirname);