code: public T headerFragment(Object model) {
          Assert.notNull(applicationContext, "Citrus application context is not initialized!");
  
          if (!CollectionUtils.isEmpty(applicationContext.getBeansOfType(Marshaller.class))) {
              return headerFragment(model, applicationContext.getBean(Marshaller.class));
          } else if (!CollectionUtils.isEmpty(applicationContext.getBeansOfType(ObjectMapper.class))) {
              return headerFragment(model, applicationContext.getBean(ObjectMapper.class));
          }
  
          throw new CitrusRuntimeException("Unable to find default object mapper or marshaller in application context");
      }
docs: Expect this message header data as model object which is marshalled to a character sequence using the default object to xml mapper that
  is available in Spring bean application context.
  
  @param model
  @return