Converting td's to th's strips colspan/rowspan

If a `td` has a colspan or rowspan, using the `containers` dropdown to convert it to a `th` strips those attributes, breaking the table format. Verified in Chrome.

For example:
```
<table>
  <tbody>
    <tr><td colspan="2"><strong>I will break</strong></td></tr>
    <tr><td>Foo</td><td>Bar</td></tr>
  </tbody>
</table>
```