Fix improper use of 'self' keyword

lib/SimpleSAML/SessionHandlerCookie.php

@@ -71,7 +71,7 @@ abstract class SessionHandlerCookie extends SessionHandler
     public function getCookieSessionId()
     {
         if ($this->session_id === null) {
-            if (self::hasSessionCookie()) {
+            if ($this->hasSessionCookie()) {
                 // attempt to retrieve the session id from the cookie
                 $this->session_id = $_COOKIE[$this->cookie_name];
             }