removeFormat: Should push down styled element into grouping element

Affected tests
- removeFormat.w3c.136
- removeFormat.w3c.136r
- removeFormat.w3c.137
- removeFormat.w3c.137r

``` html
before <div contenteditable><b><p style="font-weight: bold">foo^bar|baz</p></b></div>
after  <div contenteditable><p><b><span style="font-weight: bold">foo</span></b>^bar|<b>baz</b></p></div>
```
