title: HTMLPurifier - Change attribute value instead of remove
body: <p><code>htmlpurifier</code> removes attributes if value is not allowed. For example;</p>
  
  <pre><code>&lt;div contenteditable="true"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>I set the permissions for div tags for <code>contenteditable</code> attribute, so the value can be only <code>false</code>. But if <code>contenteditable="true"</code> or anything other than <code>false</code>, <code>htmlpurifier</code> totally removes the attribute. I don't like this behavior. I would like to have <code>htmlpurifier</code> always keep the <code>contenteditable</code> attribute and set it to <code>false</code>.</p>
  
  <p>Is this a possibility? </p>
  
  <p><strong>Actual Configs</strong></p>
  
  <pre><code>$config = \HTMLPurifier_Config::createDefault();
  
  $config-&gt;set('HTML.Allowed', 'div[contenteditable]');
  $config-&gt;set('HTML.TargetNoopener', false);
  $config-&gt;set('HTML.TargetNoreferrer', false);
  
  $def = $config-&gt;getHTMLDefinition(true);
  $def-&gt;addAttribute('div', 'contenteditable', 'Enum#false');
  </code></pre>
  
  <p><strong>Result with this configs</strong> </p>
  
  <p><em>Input:</em></p>
  
  <pre><code>&lt;div contenteditable="true"&gt;&lt;/div&gt;
  </code></pre>
  
  <p><em>Output:</em></p>
  
  <pre><code>&lt;div&gt;&lt;/div&gt;
  </code></pre>
  
  <p><strong>Desired Results</strong></p>
  
  <p><em>Input:</em></p>
  
  <pre><code>&lt;div contenteditable="true"&gt;&lt;/div&gt;
  </code></pre>
  
  <p><em>Output:</em></p>
  
  <pre><code>&lt;div contenteditable="false"&gt;&lt;/div&gt;
  </code></pre>