Endpoint that returns 400 Bad requests when coming from Zuul

endpoint.java

@RequestMapping(value = "/endpoint", method = RequestMethod.PUT, produces = "application/json")
  public Single<String> sayHello(HttpServletResponse response) throws IOException {

    InputStream inputStream = this.getClass().getClassLoader().getResourceAsStream("payload_big.json");

    Observable<String> sobs = Observable.create(new Observable.OnSubscribe<String>() {
      @Override
      public void call(Subscriber<? super String> subscriber) {
        try {
          BufferedReader br = new BufferedReader(new InputStreamReader(inputStream));

          String line;
          while((line= br.readLine())!=null) {
            subscriber.onNext(line);
          }
        } catch (Exception e) {
          subscriber.onError(e.getCause());
        } finally {
          subscriber.onCompleted();
        }
      }
    });

    return sobs.first().toSingle();
  }