def to_xml_string(str = '')
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << ('<Properties xmlns="' << APP_NS << '" xmlns:vt="' << APP_NS_VT << '">')
      instance_values.each do |key, value|
        node_name = Axlsx.camel(key)
        str << "<#{node_name}>#{value}</#{node_name}>"
      end
      str << '</Properties>'
    end
Serialize the app.xml document
@return [String]