make it possible to read the normalizerCache when inheriting

see https://github.com/FriendsOfSymfony/RestBundle/compare/lazy_normalizer_refactoring

src/Symfony/Component/Serializer/Serializer.php

@@ -30,7 +30,7 @@ class Serializer implements SerializerInterface
 {
     private $normalizers = array();
     private $encoders = array();
-    private $normalizerCache = array();
+    protected $normalizerCache = array();
 
     /**
      * @param mixed $value value to test