Parsing with XPath a xml document. Why add a <xml tag as a header> in the result?

I searched on google first and I found many result about how to parse with xpath a xml document. I have parse it but a want to convert a NODELIST in String and I have created a method for it:

private String processResult(Document responseDocument) throws XPathExpressionException,       TransformerException {
   NodeList soaphead = responseDocument.getElementsByTagName("xmlTagToTrasform"); 
   StringWriter sw = new StringWriter(); 
   Transformer serializer = TransformerFactory.newInstance().newTransformer(); 
   serializer.transform(new DOMSource(soaphead.item(0)), new StreamResult(sw)); 
   String result = sw.toString();
   return result;
}

This method works perfectly but the transformer adds an <?xml version="1.0" encoding="UTF-8"?> in the header of the result, and I don't want that. This is the result of the method:

<?xml version="1.0" encoding="UTF-8"?>
<xmlTagToTrasform>
  <xmlTagToTrasform2>
        .
        .
        .
        .
  </xmlTagToTrasform2>
</xmlTagToTrasform>