Angular 2 SpyOn provider and inject into constructor

I have a component which is using the Router in the constructor:

constructor(
      private router: Router
  ) {
      router.events.subscribe((event: any) => {
        if( event instanceof  NavigationEnd ) {
          let url: any = event.urlAfterRedirects || event.url;
          switch (url) {
            case '/login':
            case '/forgotPassword':
              this.showHeader = false;
              break;
            default:
              this.showHeader = true;
          }
        }
      });
  }

The ng2 guide on testing provides an example where it adds a 'spy' just before a method calls navigate().

However, I would like to test the component constructor:

  beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [HeaderComponent],
      providers: [
        { provide: Router,      useClass: FakeRouter },
        { provide: AuthService, useClass: null }
      ]
    });
  });

  it('should be visible on /login', inject([Router], (router: Router) => {
    let routerEventSpy = spyOn(router, 'events')
        .and.returnValue(Observable.of(new NavigationEnd(1, '/login', '/login')));
debugger;
    let fixture = TestBed.createComponent(HeaderComponent);


  }));

I thought that since Router is a singleton, if I add a spy to it in the test, and then call TestBed.createComponent which constructs the component, the component would find the spy already attached. However this isn't the case; I get :

TypeError: Cannot read property 'subscribe' of undefined

How can I attach a spy before the construction of the component, bearing in mind that I might want to construct the component multiple times for multiple tests.