sessions: use is_server_https() for secure cookie setting

include/sessions.php

@@ -12,7 +12,7 @@
 	$session_expire = min(2147483647 - time() - 1, max(SESSION_COOKIE_LIFETIME, 86400));
 	$session_name = (!defined('TTRSS_SESSION_NAME')) ? "ttrss_sid" : TTRSS_SESSION_NAME;
 
-	if ((!empty($_SERVER['HTTPS']) && ($_SERVER['HTTPS'] != 'off')) || @$_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https') {
+	if (is_server_https()) {
 		$session_name .= "_ssl";
 		ini_set("session.cookie_secure", true);
 	}