add setSerializeNull method to VisitorInterface

Serializer/VisitorInterface.php

@@ -41,6 +41,13 @@ interface VisitorInterface
      */
     function prepare($data);
 
+    /**
+     * Controls whether keys will be preserved when serializing null values
+     *
+     * @param bool $serializeNull
+     */
+    public function setSerializeNull($serializeNull);
+
     /**
      * @param mixed $data
      * @param array $type