title: @ControllerAdvice response body is always null
body: <p>I want to set the body of the response as the error message of the exception:</p>
  
  <pre><code>@ControllerAdvice
  public class RestResponseEntityExceptionHandler extends ResponseEntityExceptionHandler {
  
    @ExceptionHandler(value = EntityNotFoundException.class)
    protected ResponseEntity&lt;Object&gt; handleNotFound(EntityNotFoundException ex, WebRequest request) {
  
      System.out.println(ex.getMessage());
      return handleExceptionInternal(ex, ex.getMessage(), new HttpHeaders(), HttpStatus.I_AM_A_TEAPOT, request);
  
    }
  }
  </code></pre>
  
  <p>I'm testing it with <code>restTemplate.getForEntity(uri, String.class)</code>however all I can seem to get is a <code>418 null</code> response.</p>
  
  <p>I know execution enters here as the response is <code>418</code>, and I know the message isn't null as it's printing the value I want.</p>
  
  <p>Any help appreciated</p>