title: Add DTD when serializing with Jackson XmlMapper
body: <p>When I serialize my POJO everything works as expected. I get something like this:</p>
  
  <pre><code>&lt;?xml version='1.0' encoding='UTF-8'?&gt;
  &lt;gsafeed&gt;
     ...
  &lt;/gsafeed&gt;
  </code></pre>
  
  <p>The recipient (Google Search Appliance) seems to expect that the XML contains a DTD like this:</p>
  
  <pre><code>&lt;?xml version='1.0' encoding='UTF-8'?&gt;
  &lt;!DOCTYPE gsafeed PUBLIC "-//Google//DTD GSA Feeds//EN" ""&gt;
  &lt;gsafeed&gt;
     ...
  &lt;/gsafeed&gt;
  </code></pre>
  
  <p>How can I achieve this?</p>