add keyTypes() for User_greeting_count

plugins/Sample/User_greeting_count.php

@@ -105,6 +105,20 @@ class User_greeting_count extends Memcached_DataObject
         return array('user_id' => 'K');
     }
 
+    /**
+     * return key definitions for Memcached_DataObject
+     *
+     * Our caching system uses the same key definitions, but uses a different
+     * method to get them.
+     *
+     * @return array key definitions
+     */
+
+    function keyTypes()
+    {
+        return $this->keys();
+    }
+
     /**
      * Magic formula for non-autoincrementing integer primary keys
      *