def to_xml_string(str='')
      return if empty?
      str << '<hyperlinks>'
      each { |hyperlink| hyperlink.to_xml_string(str) }
      str << '</hyperlinks>'
    end
def to_xml_string(str='')
      return if empty?
      str << '<hyperlinks>'
      each { |hyperlink| hyperlink.to_xml_string(str) }
      str << '</hyperlinks>'
    end
seralize the collection of hyperlinks
 @return [String]