Remove heading style problematic

[Garry Yao]

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

   <h1>h[eadi]ng</h1>
   

2. Open "Format" combo and select heading 1 style to remove it from selection;
   • Actual Result:

     heading
     

   • Expected Result:

     <p>heading</p>
     

[Garry Yao]

Regression of [6983].