title: Withlatestfrom not emitting data from async observable
body: <p>as you can see from the snippet below, 
  <code>withLatestFrom</code> never completes if a promise is passed over.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>const { combineLatest, range } = rxjs;
  const { withLatestFrom } = rxjs.operators;
  
  const a$ = range(1, 5);
  const b$ = Promise.resolve('never');
  
  a$.pipe(
    withLatestFrom(b$),
  ).subscribe(console.log);</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/6.5.2/rxjs.umd.js" integrity="sha256-mNXCdYv896VtdKYTBWgurbyH+p9uDUgWE4sYjRnB5dM=" crossorigin="anonymous"&gt;&lt;/script&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>it works just fine if <code>const b$ = of(1);</code></p>
  
  <hr>
  
  <p>The documentation is not clear about this behaviour, 
  any hint?</p>