title: JSPDF-Autotable colspan/rowpan issue
body: <p>I have an html table with rowspan/colspan.I am using jspdf and jspdf-autotable for exporting that html table to pdf. However the pdf getting saved has a table which doesn't contain rowspan/colspan in actual table.How to do colspan/rowspan using jspdf-autotable?
  My current code is</p>
  
  <pre><code>&lt;html&gt;
      &lt;head&gt;
          &lt;meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /&gt;
          &lt;title id='title'&gt;HTML Page setup Tutorial&lt;/title&gt; 
          &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/jspdf/1.0.272/jspdf.debug.js"&gt;&lt;/script&gt;
  &lt;script src="https://rawgit.com/someatoms/jsPDF-AutoTable/master/dist/jspdf.plugin.autotable.js"&gt;&lt;/script&gt;
  
          &lt;script type="text/javascript"&gt;
  function myFunction()
  {
  
   var doc = new jsPDF('p', 'pt');
  
      var res = doc.autoTableHtmlToJson(document.getElementById("my-table"));
      doc.autoTable(res.columns, res.data, {startY: 40});
      doc.save("Report.pdf");
  }
  
  &lt;/script&gt;
      &lt;/head&gt;
      &lt;body&gt;
   &lt;table border='1' id="my-table"&gt;
  &lt;thead&gt;
  &lt;tr&gt;
          &lt;th&gt;A&lt;/th&gt;
          &lt;th&gt;B&lt;/th&gt;
          &lt;th&gt;C&lt;/th&gt;
  &lt;/tr&gt;
  &lt;/thead&gt;
  &lt;tbody&gt;
  &lt;tr&gt;
  &lt;td rowspan='2'&gt;D&lt;/td&gt;
  &lt;td colspan='2'&gt;$100&lt;/td&gt;
  
  &lt;/tr&gt;
  &lt;tr&gt;
  &lt;td &gt;E&lt;/td&gt;
  &lt;td &gt;F&lt;/td&gt;
  
  &lt;/tr&gt;
  &lt;/tbody&gt;
  &lt;/table&gt;
      &lt;button type="button" onclick="myFunction()"&gt;Click Me!&lt;/button&gt;
      &lt;/body&gt;
  &lt;/html&gt;  
  </code></pre>