Fixed wrong action when line start via space in webkit.

I'm sorry my english isn't good.


Error case:

In webkit, typed shift+enter (or mode is ENTER_BR, typed enter), editor generated this code:
```html
  <br>&#8203; (&#8203; is filling char.)
```
and typed space and some text, editor generated this code:
```html
  <br>&#8203; some text
```
finally, pressed enter, editor delete filling char.
```html
  <br> some text
  <br>&#8203;
```
therefore, whitespace is removed on result.

After patch:
final action changed.
```html
  <br>&nbsp;some text
  <br>&#8203;
```