title: jquery Datatable export buttons not appearing when set more than one datatable
body: <p>Export buttons work when i have a single one, but when i duplicate it, the buttons disappear.</p>
  
  <p>I'm loading:</p>
  
  <ul>
  <li>jquery.dataTables.min.css</li>
  <li>jquery.dataTables.min.js</li>
  <li>buttons.dataTables.min.css</li>
  <li>buttons.dataTables.min.js</li>
  <li>buttons.flash.min.js</li>
  <li>buttons.html5.min.js</li>
  <li>buttons.print.min.js</li>
  </ul>
  
  <p>as i read on the doc (official webpage).</p>
  
  <p>When i've a single one (doesn't matter which) it works well, but when i set both dataTables on the same page the buttons are not shown.</p>
  
  <pre><code>$(document).ready( function () {
          $('#firstTable').DataTable({
              dom: 'Bfrtip',
              buttons: [
                  'csv', 'excel', 'print'
              ]
          } );
          $('#secondTable').DataTable({
              dom: 'Bfrtip',
              buttons: [
                  'csv', 'excel', 'print'
              ]
          } );
      });
  </code></pre>
  
  <p>I think it's due to a cannot set _DT_CellIndex of undefined, which i already tried to fix but it seems that it wants to persist.
  Both tables are generated dynamically and some cell may be empty.</p>
  
  <p>I couldn't find a way to tell datatables that allow/ignore white cells.
  By the way, seems that adding an &nbsp; on void cell does not take effect.</p>
  
  <p>Any suggestion will be appreciated.</p>