def to_xml_string(node_name = '', str = '')
      str << "<#{node_name} "
      str << instance_values.map { |key, value| Axlsx::camel(key) << '="' << value.to_s << '"' }.join(' ')
      str << '/>'
    end
def to_xml_string(node_name = '', str = '')
      str << "<#{node_name} "
      str << instance_values.map { |key, value| Axlsx::camel(key) << '="' << value.to_s << '"' }.join(' ')
      str << '/>'
    end
Serialize the contenty type to xml