title: rxjs and angular router events: Set boolean to inital value after first emit
body: <p>this function below should return true if an angular router event was triggered.</p>
  <pre><code>private routeChanged$(): Observable&lt;boolean&gt; {
      return this.router.events.pipe(
          filter(event =&gt; event instanceof NavigationEnd),
          mapTo(true),
          startWith(false),
          untilDestroyed(this)
      );
  }
  </code></pre>
  <p>it works fine but it should also reset again the inital value (false) after the event was fired so
  i basically want to set the boolean back to false after it emits true the first time (when the route changed)
  (sorry for my bad english, i hope you get my point)</p>