[IE] Backspace handler on table/floated element doesn't work

[Garry Yao]

1. Load an editor with the following content and selection:

   <table border="1" cellpadding="1" cellspacing="1" style="width: 200px;">
   	<tbody>
   		<tr>
   			<td>
   				text^</td>
   		</tr>
   	</tbody>
   </table>
   

2. Try to select the whole table by clicking at the border of it;
3. Press 'Backspace';

• Actual Result: The default behavior of browser backward button is triggered.

[Garry Yao]

Already fixed in #4047.