Fix facebookarchive#529

Author: yguedidi

src/Facebook/PersistentData/PersistentDataFactory.php

@@ -54,7 +54,7 @@ public static function createPersistentDataHandler($handler)
         }
 
         if ('session' === $handler) {
-            new FacebookSessionPersistentDataHandler();
+            return new FacebookSessionPersistentDataHandler();
         }
         if ('memory' === $handler) {
             return new FacebookMemoryPersistentDataHandler();