'collapsedIcon' option in collapsible widget is not working correctly.

Hi all.

I found a matter in collapsible widget.
An abnormal operation is happened when the value of “collapsedIcon” was changed to "false" in the runtime.

If you visit here and then click ‘2-1’, you would see the malfunction.
URL : http://hyunsook.github.io/jqm-debug/latest/collapsible-collapsedIcon_false_origin.html

This PR contain the modified code about the problem.
If you visit here, you would see the normal operation.
URL : http://hyunsook.github.io/jqm-debug/latest/collapsible-collapsedIcon_false_after.html

If there are any problems or mistakes on that PR, please let me know.
Thanks in advance.