Merge branch '4.2' into 4.3

* 4.2:
  [HttpFoundation] Throw exception when the \"session\" extension is not loaded
  remove invalid test cases
  [Serializer] Fixed PHP of DenormalizableInterface::denormalize
  [Cache] work aroung PHP memory leak
  [Finder] docblock fixes
  pass error code as a string
  Catch JsonException and rethrow in JsonEncode

Author: nicolas-grekas

DependencyInjection/FrameworkExtension.php

@@ -225,6 +225,10 @@ public function load(array $configs, ContainerBuilder $container)
         }
 
         if ($this->isConfigEnabled($container, $config['session'])) {
+            if (!\extension_loaded('session')) {
+                throw new \LogicException('PHP extension "session" is required.');
+            }
+
             $this->sessionConfigEnabled = true;
             $this->registerSessionConfiguration($config['session'], $container, $loader);
             if (!empty($config['test'])) {