错误消息：
NullInjectorError: R3InjectorError(AppServerModule)
WindowRef -> InjectionToken SERVER_REQUEST_URL -> InjectionToken SERVER_REQUEST_URL -> InjectionToken SERVER_REQUEST_URL -> InjectionToken REQUEST]:
NullInjectorError: No provider for InjectionToken REQUEST!
Angular universal engine 的文档：
https://github.com/angular/universal/tree/master/modules/express-engine
服务器端的 HTTP request 调用，通过
Injection token
注入到 Application 里：
import { Request } from 'express';
import { REQUEST } from '@nguniversal/express-engine/tokens';
@Injectable()
export class RequestService {
