def to_xml_string(row_index, column_index, cell, str='')
        str << ('<c r="' << Axlsx::cell_r(column_index, row_index) << '" s="' << cell.style.to_s << '" ')
        return str << '/>' if cell.value.nil?
        method = cell.type
        self.send(method, cell, str)
        str << '</c>'
      end
Calls the proper serialization method based on type.
@param [Integer] row_index The index of the cell's row
@param [Integer] column_index The index of the cell's column
@param [String] str The string to apend serialization to.
@return [String]