fixed priority of the session listener (matches the priority set in Symfony FrameworkBundle)

src/Silex/Extension/SessionExtension.php

@@ -39,7 +39,7 @@ class SessionExtension implements ExtensionInterface
             return new NativeSessionStorage($app['session.storage.options']);
         });
 
-        $app['dispatcher']->addListener(KernelEvents::REQUEST, array($this, 'onKernelRequest'), -255);
+        $app['dispatcher']->addListener(KernelEvents::REQUEST, array($this, 'onKernelRequest'), 128);
 
         if (!isset($app['session.storage.options'])) {
             $app['session.storage.options'] = array();