title: extjs: write an xml without root using xmlwriter
body: <p>when sending a response to a server using xmlwriter, the root tag will always be added to the request:</p>
  
  <p>If I want to post an item:</p>
  
  <pre><code>&lt;book&gt;
  &lt;id&gt;&lt;/id&gt;
  &lt;title&gt;&lt;/title&gt;
  &lt;publisher&gt;&lt;/publisher&gt;
  &lt;/book&gt;
  </code></pre>
  
  <p>the actual generated xml is:</p>
  
  <pre><code>&lt;xmlData&gt;
  &lt;book&gt;
  &lt;id&gt;&lt;/id&gt;
  &lt;title&gt;&lt;/title&gt;
  &lt;publisher&gt;&lt;/publisher&gt;
  &lt;/book&gt;
  &lt;/xmlData&gt;
  </code></pre>
  
  <p>I can make sure that only one record will be posted: one book for each request</p>
  
  <p>how can I remove the xmlData element? that is to say, make book tag to be the root element</p>
  
  <p>the extjs document says:</p>
  
  <p>Ext.data.writer.Xml
  documentRoot : String</p>
  
  <p>The name of the root element of the document. Defaults to 'xmlData'. If there is <strong>more than 1 record and the root is not specified</strong>, the default document root will still be used to ensure a valid XML document is created.</p>
  
  <p>My understanding is that if <strong>there is only one record</strong>, the default document root will not be used. Am I right? But acturaly, the root element 'xmlData' is still added even if there is only one record.</p>
  
  <p>Thanks</p>