def to_xml
      out = ""
      builder = Builder::XmlMarkup.new(:target => out, :indent => 2)
      builder.instruct!
      build_xml(builder)
      out
    end
def to_xml
      out = ""
      builder = Builder::XmlMarkup.new(:target => out, :indent => 2)
      builder.instruct!
      build_xml(builder)
      out
    end
Convert to xml of container item

 @return [String] the xml of container item