[AsseticBundle] fixed unit tests

src/Symfony/Bundle/AsseticBundle/Tests/DependencyInjection/AsseticExtensionTest.php

@@ -59,6 +59,7 @@ class AsseticExtensionTest extends \PHPUnit_Framework_TestCase
         $this->container->setParameter('kernel.cache_dir', __DIR__);
         $this->container->setParameter('kernel.debug', false);
         $this->container->setParameter('kernel.root_dir', __DIR__);
+        $this->container->setParameter('kernel.charset', 'UTF-8');
         $this->container->set('kernel', $this->kernel);
     }