def to_xml_string(str='')
      return unless range
      str << "<autoFilter ref='#{range}'>"
      columns.each { |filter_column| filter_column.to_xml_string(str) }
      str << "</autoFilter>"
    end
def to_xml_string(str='')
      return unless range
      str << "<autoFilter ref='#{range}'>"
      columns.each { |filter_column| filter_column.to_xml_string(str) }
      str << "</autoFilter>"
    end
serialize the object
 @return [String]