title: Cancel observable after first time - Angular2, TypeScript, rxjs/Observable
body: <p>I’m doing a check for existing login at start of app which I want only once.  How do we cancel that subscription after the first time?  I tried take(1) but that doesn't seem to work.  </p>
  
  <pre><code>this.auth.getUserData().take(1).subscribe(data =&gt; { // the take(1) doesn't work
    console.log('Firebase responded with success.');
    this.rootPage = TabsPage;
   }, err =&gt; {
    console.log('Firebase responded with error.', err);
    this.rootPage = LoginEmailPage;
   }
  );
  </code></pre>