Invalid table headers after switching from both to column

[Marek Lewandowski]

1. Open any sample with CKEditor. (i.e. samples/replacebyclass.html)
2. Use following source.

   <table border="1" cellpadding="1" cellspacing="1" style="width:500px" summary="bar">
   	<caption>foo</caption>
   	<thead>
   		<tr>
   			<th scope="col">aa</th>
   			<th scope="col">aa</th>
   			<th scope="col">aa</th>
   		</tr>
   	</thead>
   	<tbody>
   		<tr>
   			<th scope="row">bb</th>
   			<td>cc</td>
   			<td>dd</td>
   		</tr>
   		<tr>
   			<th scope="row">bb</th>
   			<td>cc</td>
   			<td>dd</td>
   		</tr>
   	</tbody>
   </table>
   

3. Right click at table, and use "Table Properties" menu item.
4. With "Headers" dropdown, set its value to "First Column".
5. Accept dialog by clicking OK.

Expected result:
First column should contain headers.

Current result:
There is no header (th element) in first row.

Additional info:

1. Tested with Chromium 42.0.2296.0.

[Jakub Ś]

DUP of #9189