New lines at the end of list items are not preserved by the wysiwyg

To reproduce, type this in wiki:

* \\
** \\
*** test1

edit in wysiwyg, save and then edit back in wiki, the new lines in the first list items are no longer there.
This is due to the fact that we mark all br unnecessary brs and strip them out when saving in wysiwyg, and this is a case when the br is unnecessary.

It currently reproduces only in firefox, because stripping code only runs for firefox.