title: TableTools only exporting single header row
body: <p>I am using the <a href="http://datatables.net/" rel="nofollow">DataTables jquery plugin</a> </p>
  
  <p>I have a DataTable that has multi-row table header with colspan. 
  Something like:</p>
  
  <pre><code>&lt;thead&gt;
    &lt;tr&gt;
      &lt;th&gt;Level 1&lt;/th&gt;
      &lt;th colspan='2'&gt;Level 1 - Item 1&lt;/th&gt;
      &lt;th colspan='2'&gt;Level 1 - Item 2&lt;/th&gt;
    &lt;/tr&gt;
    &lt;tr&gt;
      &lt;th&gt;Level 2&lt;/th&gt;
      &lt;th&gt;Level 2 - Item 1a&lt;/th&gt;
      &lt;th&gt;Level 2 - Item 1b&lt;/th&gt;
      &lt;th&gt;Level 2 - Item 2a&lt;/th&gt;
      &lt;th&gt;Level 2 - Item 2b&lt;/th&gt;
    &lt;/tr&gt;
  &lt;/thead&gt;
  </code></pre>
  
  <p>However when I use the TableTools plugin to export then except for the "Print" option all the rest (Excel, CSV, Pdf etc) only has the "Level 2" header row and not the Level 1.</p>
  
  <p>Any suggestions on how to get it to export also Level 1? </p>