RESTeasy: Cannot configure Jackson ObjectMapper via ContextResolver nor Application.getSingletons()

@Provider @Produces(MediaType.APPLICATION_JSON)
public class JacksonContextResolver implements ContextResolver<ObjectMapper> {

	private transient Logger log = LoggerFactory.getLogger(JacksonContextResolver.class);
	private ObjectMapper objectMapper;

	public JacksonContextResolver() throws JsonGenerationException, JsonMappingException, IOException {
		log.info("Using my own Jackson ObjectMapper");
		objectMapper = new ObjectMapper();
		objectMapper.configure(Feature.INDENT_OUTPUT, true);
		objectMapper.configure(Feature.WRITE_DATES_AS_TIMESTAMPS, false);
		objectMapper.configure(Feature.WRITE_DATE_KEYS_AS_TIMESTAMPS, false);
		
		log.info("Test date: {}", objectMapper.writeValueAsString(new DateTime()));
		log.info("Test pretty output: {}", objectMapper
				.writeValueAsString(new Article("Launching AksiMata",
						"AksiMata : Ada apa di sana?",
						"Webapp yang keren dan oke punya.", "Hendy Irawan")));
	}
	
	@Override
	public ObjectMapper getContext(Class<?> type) {
		return objectMapper;
	}
}

@Override
public Set<Object> getSingletons() {
	Set<Object> singletons = new HashSet<Object>(super.getSingletons());
	ObjectMapper objectMapper = new ObjectMapper();
	objectMapper.configure(Feature.INDENT_OUTPUT, true);
	objectMapper.configure(Feature.WRITE_DATES_AS_TIMESTAMPS, false);
	objectMapper.configure(Feature.WRITE_DATE_KEYS_AS_TIMESTAMPS, false);
	singletons.add(objectMapper);
	return singletons;
}