code: public HttpHeaders generatePreflightResponseHeaders() {
          final HttpHeaders headers = new DefaultHttpHeaders();
          preflightResponseHeaders.forEach((key, value) -> {
              final Object val = getValue(value);
              if (val instanceof Iterable) {
                  headers.addObject(key, (Iterable<?>) val);
              } else {
                  headers.addObject(key, val);
              }
          });
          return headers.asImmutable();
      }
docs: Generates immutable HTTP response headers that should be added to a CORS preflight response.
  
  @return {@link HttpHeaders} the HTTP response headers to be added.