def serialized_attributes(str = '', additional_attributes = {})
      attributes = declared_attributes.merge! additional_attributes
      attributes.each do |key, value|
        str << "#{Axlsx.camel(key, false)}=\"#{Axlsx.camel(Axlsx.booleanize(value), false)}\" "
      end
      str
    end
serializes the instance values of the defining object based on the
list of serializable attributes.
@param [String] str The string instance to append this
serialization to.
@param [Hash] additional_attributes An option key value hash for
defining values that are not serializable attributes list.