title: Symfony2 : Why getToken return null when injecting SecurityContext in a TwigExtension?
body: <p>I did exactly the answer from <a href="https://stackoverflow.com/questions/9990625/symfony2-inject-current-user-in-service">this post</a> but the token property is null and the user is correctly logged in and the route is behind a firewall. Also, I am injecting the SecurityContext in other services and it works fine. </p>
  
  <p>services.xml :</p>
  
  <pre><code>&lt;service id="tc.extensions.relation_helper"
   class="TC\CoreBundle\Extensions\RelationHelperExtension"&gt;
      &lt;argument type="service" id="security.context" /&gt;
      &lt;tag name="twig.extension" /&gt;
  &lt;/service&gt;
  </code></pre>
  
  <p>My extension:</p>
  
  <pre><code>class RelationHelperExtension extends Twig_Extension
  {
      /**
       * @var User 
       */
      private $user;
  
      public function __construct(SecurityContext $securityContext){
          $this-&gt;user = $securityContext-&gt;getToken()-&gt;getUser();
      }
  </code></pre>