title: How to add a css class on a "form_row" in twig template
body: <p>I would like to know how I can add a css class on a "{{ form_row() }}" in twig. For the moment, I have this code :</p>
  
  <pre><code>{{ form_row(form.username, {'label' : "Login", 'attr': {'class': 'loginForm'}}) }}
  </code></pre>
  
  <p>... But the CSS class "loginForm" isn't used in the HTML code.</p>
  
  <p>Thank you :) ! </p>