title: Adding attribute to XML Node always fail
body: <p>I always get exception below when try to add attribute, why it's not working?</p>
  
  <blockquote>
    <p>The prefix '' cannot be redefined from '' to '<a href="http://ws.plimus.com" rel="nofollow">http://ws.plimus.com</a>'
    within the same start element tag.</p>
  </blockquote>
  
  <p>Code</p>
  
  <pre><code>var docXml = new XElement("param-encryption", 
      new XAttribute("xmlns", "http://ws.plimus.com"),
      new XElement("parameters"));
  
  var s = docXml.ToString();
  </code></pre>
  
  <p>I want to get result like</p>
  
  <pre><code>&lt;param-encryption xmlns="http://ws.plimus.com"&gt;
    &lt;parameters&gt;
  
  
    &lt;/parameters&gt;
  &lt;/param-encryption&gt;
  </code></pre>