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

The button widget doesn't set the iconpos option, like the bug of the icon option (#6625).

Here are test pages.
* origin: http://hyunsook.github.io/jqm-debug/latest/buttons-button_setoptions_iconpos_origin.html
* modified: http://hyunsook.github.io/jqm-debug/latest/buttons-button_setoptions_iconpos_after.html

NOTE: The codes were modified by basing on the c28c02cc6b commit of master, not the commit for #6625.