Please override the $.fn.combobox.parseData function to solve your issue.
$.fn.combobox.parseData = function(target){
var data = [];
var opts = $(target).combobox('options');
$(target).children('option').each(function(){
var item = {};
item[opts.valueField] = $(this).attr('value')!=undefined ? $(this).attr('value') : $(this).text();
item[opts.textField] = $(this).text();
item['selected'] = $(this).attr('selected');
data.push(item);
});
return data;
};