code: public HttpHeaders preflightResponseHeaders() {
          if (preflightHeaders.isEmpty()) {
              return EmptyHttpHeaders.INSTANCE;
          }
          final HttpHeaders preflightHeaders = new DefaultHttpHeaders();
          for (Entry<CharSequence, Callable<?>> entry : this.preflightHeaders.entrySet()) {
              final Object value = getValue(entry.getValue());
              if (value instanceof Iterable) {
                  preflightHeaders.add(entry.getKey(), (Iterable<?>) value);
              } else {
                  preflightHeaders.add(entry.getKey(), value);
              }
          }
          return preflightHeaders;
      }
docs: Returns HTTP response headers that should be added to a CORS preflight response.
  
  @return {@link HttpHeaders} the HTTP response headers to be added.