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