title: BehaviorSubject map a BehaviorSubject
body: <p>I have a rxjs@6 BehaviorSubject <code>source$</code>, I want get subvalue from <code>source$</code></p>
  
  <pre><code>  const source$ = new BehaviorSubject(someValue);
    const subSource$ = source$.pipe(map(transform));
  </code></pre>
  
  <p>I expect the <code>subSource$</code> also is a BehaviorSubject, but is not and How can I get the <code>subSource$</code> is a BehaviorSubject ?</p>