title: Change role of one user no working with Symfony2
body: <p>Sorry if this question has been asked many times but I didn't found any solution on the Web.
  So, I just have installed FOSUserBundle and I've created 2 users on my application with the console. The problem is they don't have any role.
  So in on controller, I get the user Entity and y set his role but when I try to access to a page restricted for admin, it says access denied. Moreover, the roles has no changed on the databse.
  My code in the controller : </p>
  
  <pre><code>$user = $this-&gt;container-&gt;get('security.context')-&gt;getToken()-&gt;getUser();
  $user-&gt;setRoles('ROLE_ADMIN');
  if (!$this-&gt;get('security.context')-&gt;isGranted('ROLE_ADMIN')){
      throw new AccessDniedException('Access Denied !');
  }
  </code></pre>
  
  <p>Anyone have an idea ? 
  Thanks in advance !</p>