title: Symfony2 form widget container attributes
body: <p>I need set a class for widget container, but have no idea how Symfony passes attributes to "widget_container_attributes" block</p>
  
  <p>Widget container template:</p>
  
  <pre><code>{% block form_widget %}
  {% spaceless %}
      &lt;div {{ block('widget_container_attributes') }}&gt;
          {{ block('field_rows') }}
          {{ form_rest(form) }}
      &lt;/div&gt;
  {% endspaceless %}
  {% endblock form_widget %}
  </code></pre>
  
  <p>Example result:</p>
  
  <pre><code>&lt;div class="MY-CLASS"&gt;
      &lt;label class="required" for="page_title"&gt;Title&lt;/label&gt;
      &lt;input type="text" value="Next subpage" maxlength="500" required="required" name="page[title]" id="page_title"&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>how can I do that?</p>