title: XDocument.ToString() drops XML Encoding Tag
body: <p>Is there any way to get the xml encoding in the toString() Function?</p>
  
  <p><strong>Example:</strong></p>
  
  <pre><code>xml.Save("myfile.xml");
  </code></pre>
  
  <p>leads to</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;Cooperations&gt;
    &lt;Cooperation&gt;
      &lt;CooperationId&gt;xxx&lt;/CooperationId&gt;
      &lt;CooperationName&gt;Allianz Konzern&lt;/CooperationName&gt;
      &lt;LogicalCustomers&gt;
  </code></pre>
  
  <p>But </p>
  
  <pre><code>tb_output.Text = xml.toString();
  </code></pre>
  
  <p>leads to an output like this</p>
  
  <pre><code>&lt;Cooperations&gt;
    &lt;Cooperation&gt;
      &lt;CooperationId&gt;xxx&lt;/CooperationId&gt;
      &lt;CooperationName&gt;Allianz Konzern&lt;/CooperationName&gt;
      &lt;LogicalCustomers&gt;
      ...
  </code></pre>