[IE] enterkey doesn't create new list item

[garry.yao]

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

   <ol>
   	<li>
   		<h1>heading list^</h1>
   	</li>
   </ol>
   

2. Press enter at the end;

• Actual Result: New paragraph created instead of new list item.

[garry.yao]

Root same cause with #6807.

[garry.yao]

Fixed by #6807.

[james c]

There is still an issue here in IE8 non-compatibility mode.

Test Case:

Follow the 2 steps in the test case above.

Notice after step 2 that the cursor appears below the number 2 intead of beside it. (incorrect behaviour)

Now press backspace.

Expected: The number 2 is deleted & the cursor returns to the end of the first list item.

Actual: The number 2 is replaced by a number 1 in Heading 1 paragraph format & the cursor returns to the end of the first list item.

(Also try clicking after the last list item & pressing backspace. You cannot delete the number)