code: private List<T> unmarshallResponseHeaderToList(
              JsonUnmarshallerContext context) throws Exception {
  
          String headerValue = context.readText();
  
          List<T> list = new ArrayList<T>();
  
          String[] headerValues = headerValue.split("[,]");
  
          for (final String headerVal : headerValues) {
              list.add(itemUnmarshaller.unmarshall(new JsonUnmarshallerContext() {
                  @Override
                  public String readText() {
                      return headerVal;
                  }
              }));
          }
          return list;
      }
docs: Un marshalls the response header into the list.