title: Symfony2 FosUser : CSRF token issue
body: <p>I use the authentification system provided by FOSUser bundle</p>
  
  <p>Everything was doing fine but recently when clicking on : </p>
  
  <pre><code>     &lt;form action="{{ path('fos_user_registration_register') }}" method="post"&gt;                          
                             &lt;button class="btn btn-small btn-success" type="submit"&gt;S'inscrire&lt;/button&gt;                     
       &lt;/form&gt;  
  </code></pre>
  
  <p>that generated the error: The CSRF token is invalid. Please try to resubmit the form.</p>
  
  <p>none of the view nor controller of FOSUserBundle have being modified...!?</p>
  
  <p>How come this happened and how can this be fixed?</p>