prevent remove empty span

hi

i'm going to upgrade to 4.1.1 but 

CKEDITOR.dtd.$removeEmpty['span'] = 0;

not working. it works in ver 3.6.

my code in SOURCE mode :

<span id="test"></span>

<span></span>

<span></span>

<span id="test">&nbsp</span>

<span>&nbsp</span>

Same Result after back in to design mode and source is empty

any idea?