Button: Fixed the bugs of _setOptions - The button widget doesn't set the iconshadow option.

The button widget doesn't set the iconshadow option, like the bugs of the icon option (#6625) and the iconpos option (#6626).

Here are test pages.
* origin: http://hyunsook.github.io/jqm-debug/latest/buttons-button_setoptions_iconshadow_origin.html
* modified: http://hyunsook.github.io/jqm-debug/latest/buttons-button_setoptions_iconshadow_after.html

NOTE: The codes were modified by basing on the c28c02c commit of master, not the commits for #6625 or #6626. 

I already know that the iconshadow option is deprecated in 1.4., so I don't know whether that PR will be helpful for you, or not. I hope that my PR can help you, but if the PR will not be helpful, you can close.