Adding set to autoloader.

Fixes #2319

lib/Cake/Model/Model.php

@@ -25,6 +25,7 @@
 App::uses('ClassRegistry', 'Utility');
 App::uses('Validation', 'Utility');
 App::uses('String', 'Utility');
+App::uses('Set', 'Utility');
 App::uses('BehaviorCollection', 'Model');
 App::uses('ModelBehavior', 'Model');
 App::uses('ConnectionManager', 'Model');