title: jsPDF html method always fails
body: <p>I am trying to generate a PDF from an html element (a div) with some content:</p>
  
  <pre><code>&lt;div id = "toPDF"&gt;
  &lt;table &gt;
    &lt;thead&gt;
      &lt;tr&gt;
        &lt;th&gt;Destination&lt;/th&gt;
        &lt;th&gt;Start Date&lt;/th&gt;
        &lt;th&gt;End Date&lt;/th&gt;
        &lt;th&gt;Comment&lt;/th&gt;
      &lt;/tr&gt;
    &lt;/thead&gt;
    &lt;tbody&gt;
      &lt;tr&gt;
        &lt;td&gt;Dest1&lt;/td&gt;
        &lt;td&gt;Date1&lt;/td&gt;
        &lt;td&gt;Date2&lt;/td&gt;
        &lt;td&gt;Comment1&lt;/td&gt;
      &lt;/tr&gt;
    &lt;/tbody&gt;
  &lt;/table&gt;
  &lt;/div&gt;
  
  .
  .
  .
  &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/html2canvas/0.4.1/html2canvas.js"&gt;&lt;/script&gt;
  &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/jspdf/1.5.3/jspdf.debug.js" integrity="sha384-NaWTHo/8YCBYJ59830LTz/P4aQZK1sS0SneOgAvhsIl3zBu8r9RevNg5lHCHAuQ/" crossorigin="anonymous"&gt;&lt;/script&gt;
  &lt;script src="app.js"&gt;&lt;/script&gt;
  
  </code></pre>
  
  <p>I am using the following javascript:</p>
  
  <pre><code>window.html2canvas = html2canvas;
  
  var elem = document.getElementById('toPDF');
  
  pdf.html(elem,
      {
          x: 20,
          y: 140,
          callback:
              function (pdf) {
                  pdf.text(80, 230, "Test Text");
                  var iframe = document.createElement('iframe');
                  iframe.setAttribute('style', 'position:absolute;top:0;right:0;height:100%; width:600px');
                  document.body.appendChild(iframe);
                  iframe.src = pdf.output('datauristring');
              }
      });
  
  </code></pre>
  
  <p>The resulting PDF shows the "Test Text", but no trace of the html table. If I remove "Test Text" the output PDF is empty. The browser console shows the following error:</p>
  
  <pre><code>TypeError: Argument 1 of CanvasRenderingContext2D.drawImage could not be converted to any of: HTMLImageElement, SVGImageElement, HTMLCanvasElement, HTMLVideoElement, ImageBitmap
  </code></pre>
  
  <p>I have tried different options. I am asking for a working jsPDF.html() example.</p>