def to_xml_string(str = '')
      str << '<gradientFill '
      serialized_attributes str
      str << '>'
      @stop.each { |s| s.to_xml_string(str) }
      str << '</gradientFill>'
    end
def to_xml_string(str = '')
      str << '<gradientFill '
      serialized_attributes str
      str << '>'
      @stop.each { |s| s.to_xml_string(str) }
      str << '</gradientFill>'
    end
Serializes the object
 @param [String] str
 @return [String]