code: private Mono<Void> cleanup(@Nullable Throwable cause) {
          if (future.isDone()) {
              return Mono.empty();
          }
  
          if (cause != null) {
              future.completeExceptionally(cause);
              logger.debug("{} Response future has been completed with a cause", ctx, cause);
              return Mono.empty();
          }
  
          final HttpResponse response = HttpResponse.of(armeriaHeaders);
          future.complete(response);
          logger.debug("{} Response future has been completed with an HttpResponse", ctx);
          return Mono.fromFuture(response.completionFuture());
      }
docs: Closes the {@link HttpResponseWriter} if it is opened.