title: HTMLPurifier allow class attribute
body: <p>How can i allow "class" in HTMLPurifier? I am trying to purify this:</p>
  
  <pre><code> &lt;div class="txt_r" id="test"&gt;Blah&lt;/div&gt;
  </code></pre>
  
  <p>And i get:</p>
  
  <pre><code> &lt;div id="test"&gt;Blah&lt;/div&gt;
  </code></pre>
  
  <p>Why class is dissapeared? I am using next config:</p>
  
  <pre><code> $config-&gt;set('Attr.EnableID', true);
   $config-&gt;set('CSS.Trusted', true);
   $config-&gt;set('HTML.AllowedAttributes', 'style, src, class');
  </code></pre>