Can't add optional class to menu item when using Walker_Nav_Menu

As a wordpress beginner. I’m creating menu also want to customized the <li> tag to be able to assign different class name to control specific behavior through CSS so I use custom walker to get a list with clean markup that I found here:

https://gist.github.com/toscho/1053467

The code work perfectly. But now, when I want to add optional class to one item in menu via “Css classes” but it doesn’t work.