fixed translator

As of Symfony 2.5.2, Translator::__construct() calls setLocale.
As the Silex Translator calls it with null and because we synchronize
the translator language with the app one, the locale was always
set to null when calling the translator for the first time, overriding
the current app locale.

src/Silex/Translator.php

@@ -37,6 +37,10 @@ class Translator extends BaseTranslator
 
     public function setLocale($locale)
     {
+        if (null === $locale) {
+            return;
+        }
+
         $this->app['locale'] = $locale;
 
         parent::setLocale($locale);