def to_xml_string(str = '')
      valid_attributes = get_valid_attributes

      str << '<dataValidation '
      str << instance_values.map do |key, value| 
        '' << key << '="' << Axlsx.booleanize(value).to_s << '"' if (valid_attributes.include?(key.to_sym) && !CHILD_ELEMENTS.include?(key.to_sym)) 
      end.join(' ')
      str << '>'
      str << ('<formula1>' << self.formula1 << '</formula1>') if @formula1 and valid_attributes.include?(:formula1)
      str << ('<formula2>' << self.formula2 << '</formula2>') if @formula2 and valid_attributes.include?(:formula2)
      str << '</dataValidation>'
    end
Serializes the data validation
@param [String] str
@return [String]