def to_xml_string(str = '')
      str << '<sheetData>'
      worksheet.rows.each_with_index do |row, index| 
        row.to_xml_string(index, str) 
      end
      str << '</sheetData>'
    end
Serialize the sheet data
@param [String] str the string this objects serializaton will be concacted to.
@return [String]