title: mergeMap throws 'invalid object where a stream was expected' error in rxjs6
body: <p>After migrating rxjs from v5 to v6 I experience an error when using mergeMap: </p>
  
  <pre><code>of('foobar').pipe(
    mergeMap(() =&gt; of('baz'))
  ).subscribe(console.log);  
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>core.js:1598 ERROR TypeError: You provided an invalid object where a stream was expected. You can provide an Observable, Promise, Array, or Iterable.
      at Object.push../node_modules/rxjs/internal/util/subscribeTo.js.exports.subscribeTo (subscribeTo.js:42)
      at Object.subscribeToResult (subscribeToResult.js:7)
      at MergeMapSubscriber.push../node_modules/rxjs/internal/operators/mergeMap.js.MergeMapSubscriber._innerSub (mergeMap.js:132)
      at MergeMapSubscriber.push../node_modules/rxjs/internal/operators/mergeMap.js.MergeMapSubscriber._tryNext (mergeMap.js:129)
      at MergeMapSubscriber.push../node_modules/rxjs/internal/operators/mergeMap.js.MergeMapSubscriber._next (mergeMap.js:112)
      at MergeMapSubscriber.push../node_modules/rxjs/internal/Subscriber.js.Subscriber.next (Subscriber.js:103)
      at Observable._subscribe (scalar.js:5)
      at Observable.push../node_modules/rxjs/_esm5/internal/Observable.js.Observable._trySubscribe (Observable.js:176)
      at Observable.push../node_modules/rxjs/_esm5/internal/Observable.js.Observable.subscribe (Observable.js:161)
      at MergeMapOperator.push../node_modules/rxjs/internal/operators/mergeMap.js.MergeMapOperator.call (mergeMap.js:87)
  </code></pre>
  
  <p><code>of('baz')</code> is a stream, right? So why do I get this error? </p>