def to_xml_string(r_index, str = '')
      serialized_tag('row', str, :r => r_index + 1) do
        tmp = '' # time / memory tradeoff, lots of calls to rubyzip costs more
                 # time..
        each_with_index { |cell, c_index| cell.to_xml_string(r_index, c_index, tmp) }
        str << tmp
      end
    end
Serializes the row
@param [Integer] r_index The row index, 0 based.
@param [String] str The string this rows xml will be appended to.
@return [String]