title: dataSet.GetXml() doesn't return xml for null or blank columns
body: <p>When I call dataSet.GetXml() I don't get any xml returned for columns with null or blank values. Is there a simple, efficient way to get around this? An example of the problem below. Notice how a2 is missing from the second results section. </p>
  
  <pre><code>&lt;results&gt;
  &lt;a1&gt;test1&lt;/a1&gt;
  &lt;a2&gt;test2&lt;/a2&gt;
  &lt;a3&gt;test3&lt;/a3&gt;
  &lt;/results&gt;
  &lt;results&gt;
  &lt;a1&gt;Atest1&lt;/a1&gt;
  &lt;a3&gt;Atest3&lt;/a3&gt;
  &lt;/results&gt;
  </code></pre>