code: public T headerFragment(Object model, String mapperName) {
          Assert.notNull(applicationContext, "Citrus application context is not initialized!");
  
          if (applicationContext.containsBean(mapperName)) {
              Object mapper = applicationContext.getBean(mapperName);
  
              if (Marshaller.class.isAssignableFrom(mapper.getClass())) {
                  return headerFragment(model, (Marshaller) mapper);
              } else if (ObjectMapper.class.isAssignableFrom(mapper.getClass())) {
                  return headerFragment(model, (ObjectMapper) mapper);
              } else {
                  throw new CitrusRuntimeException(String.format("Invalid bean type for mapper '%s' expected ObjectMapper or Marshaller but was '%s'", mapperName, mapper.getClass()));
              }
          }
  
          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 given object to xml mapper that
  is accessed by its bean name in Spring bean application context.
  
  @param model
  @param mapperName
  @return