def sorting_html_class(sort_on)
      return TableBuilder.html_sorting_class.first if current_sorting(sort_on).nil?
      return TableBuilder.html_sorting_class.second if current_sorting(sort_on) == :asc
      TableBuilder.html_sorting_class.third
    end
def sorting_html_class(sort_on)
      return TableBuilder.html_sorting_class.first if current_sorting(sort_on).nil?
      return TableBuilder.html_sorting_class.second if current_sorting(sort_on) == :asc
      TableBuilder.html_sorting_class.third
    end
Return a string with html class of sorting for headers
 The html class is based on option: SortingTableFor::TableBuilder.html_sorting_class