def string(cell, str='')
        if cell.is_array_formula?
          array_formula_serialization cell, str
        elsif cell.is_formula?
          formula_serialization cell, str
        elsif !cell.ssti.nil?
          value_serialization 's', cell.ssti, str
        else
          inline_string_serialization cell, str
        end
      end
Serializes cells that are type string
@param [Cell] cell The cell that is being serialized
@param [String] str The string the serialized content will be appended to.
@return [String]