Spring REST service hangs up when forwarding a ResponseEntity

Question

I am facing a problem using a REST service made up with Spring Boot 1.5. I am developing a REST service that acts as a proxy, forwarding requests to another REST service that exposes the same API.

@RestController
@RequestMapping("/user")
public class ProxyUserController {

    // Some initialization code

    @PostMapping
    public ResponseEntity<?> add(@Valid @RequestBody User user) {
        return restTemplate.postForEntity(userUrl, user, String.class);
    }

    @Configuration
    public static class RestConfiguration {
        @Bean
        public RestTemplate restTemplate(UserErrorHandler errorHandler) {
            return new RestTemplateBuilder().errorHandler(errorHandler).build();
        }
    }

    @Component
    public static class UserErrorHandler implements ResponseErrorHandler {
        @Override
        public boolean hasError(ClientHttpResponse response) {
            return false;
        }
        @Override
        public void handleError(ClientHttpResponse response) {
            // Empty body
        }
    }
}

As you can see, to avoid that RestTemplate surrounds any error response with an exception that causes the creation of a new response with status 500, I defined a customer ResponseErrorHandler.

The problem I faced is that if the postForEntity returns a response with an HTTP Status different from 200, the response will never arrive at the caller, that hangs up until the timeout hits him.

However, if I create a new response starting from the one returned by the postForEntity, all starts to work smoothly.

@PostMapping
public ResponseEntity<?> add(@Valid @RequestBody User user) {
    final ResponseEntity<?> response = 
        restTemplate.postForEntity(userUrl, user, String.class);
    return ResponseEntity.status(response.getStatusCode()).body(response.getBody());
}

What the hell is going on? Why I can't reuse a ResponseEntity coming from another call?

Thanks to all.