code: public static Middleware<AsyncHandler<?>, AsyncHandler<Response<?>>> replyContentType(
        String contentType) {
  
      return inner -> inner
          .map(Middlewares::ensureResponse)
          .map(response -> response.withHeader(CONTENT_TYPE, contentType));
    }
docs: Middleware that adds the ability to set the response's Content-Type header to a defined
  value.
  
  This middleware is type-unsafe, and it might be a better idea to set the content type directly
  in your own middleware that does response serialization.