Selectmenu: Does not skip disabled items when menu is open

The Selectmenu widget does not skip disabled items when the menu is open and the user uses the up- and down-arrow keys to move through the items. This is inconsistent with the behavior of a native browser <select> element, which does skip the disabled items (as tested on Windows in IE, Firefox, and Chrome).

This jsbin example is a simple comparison of a native <select> element and one with Selectmenu turned on: https://jsbin.com/lulorasuza/edit?html,js,output

The issue is related to this code in the _move function:

		if ( this.isOpen ) {
			item = this.menuItems.eq( this.focusIndex );
		} else {
			item = this.menuItems.eq( this.element[ 0 ].selectedIndex );
			filter += ":not(.ui-state-disabled)";
		}

Note that it is explicitly only filtering out .ui-state-disabled if the Selectmenu is not open. If this filter were set unconditionally, the behavior would be consistent with the native <select>.