New list item should not copy the value attribute from previous list item

[Wiktor Walc]

Right now there is no way to set the value attribute, but after closing #7131 we will have this problem.

Steps to reproduce

1. Start with

   <ol>
   	<li value="7">
   		Foo</li>
   </ol>
   

2. Press enter.
3. Result:

   <ol>
   	<li value="7">
   		Foo</li>
   	<li value="7">
   		&nbsp;</li>
   </ol>
   

4. Expected result:

   <ol>
   	<li value="7">
   		Foo</li>
   	<li>
   		&nbsp;</li>
   </ol>
   

[Sa'ar Zac Elias]

Fixed with [6552].