code: @Override
    public String asXML(T instance) throws JAXBException {
      Marshaller marshaller = getMarshaller(instance);
      marshaller.setProperty(MarshallerProperties.MEDIA_TYPE, "application/xml");
      String result = getString(marshaller, instance);
      return result;
    }
docs: create an xml representation for the given <T> instance
  
  @param instance
  - the instance to convert to xml
  @return a xml representation of the given <T> instance
  @throws JAXBException