title: DataTables throwing error when colspan in table header cell
body: <p>I'm getting the following error when using a colspan in my th tag.</p>
  
  <pre><code>Uncaught TypeError: Cannot read property 'mData' of undefined
  </code></pre>
  
  <p>Normally this error occurs when the number of table cells is not equal in every row, but this is not the case. Here is my code:</p>
  
  <pre><code>&lt;table id="foo"&gt;
  &lt;thead&gt;
      &lt;tr&gt;
          &lt;th colspan="5"&gt;asdf&lt;/th&gt;
      &lt;/tr&gt;
  &lt;/thead&gt;
  
  &lt;tbody&gt;
  
      &lt;tr&gt;
          &lt;td&gt;1&lt;/td&gt;
          &lt;td&gt;2&lt;/td&gt;
          &lt;td&gt;3&lt;/td&gt;
          &lt;td&gt;4&lt;/td&gt;
          &lt;td&gt;5&lt;/td&gt;
      &lt;/tr&gt;
  
  &lt;/tbody&gt;
  &lt;/table&gt;
  
  &lt;script&gt;
      $('#foo').DataTable();
  &lt;/script&gt;
  </code></pre>
  
  <p>Here is an example in codepen: <a href="https://codepen.io/anon/pen/EQOVMe?editors=1111" rel="nofollow noreferrer">https://codepen.io/anon/pen/EQOVMe?editors=1111</a></p>
  
  <p>Using jquery 1.12.3 and Data Tables 1.10.16</p>
  
  <p>Any ideas?</p>