fix: add test from #5932 and plug leak in share

[cartant]

Description:

This PR brings in the test from #5932 and fixes a leak that it exposed in the share implementation.

Maybe we should add the test to the specs for share, too, but this is all I have time for, ATM.

Related PR: #5932

[benlesh]

@cartant I've added another commit that more appropriately fixes the issue.

The problem was we were accidentally returning a teardown that was basically adding a closure over a subscriber to itself.

Effectively:

new Observable(subscriber => {
  return () => {
    subscriber.unsubscribe();
  };
});

It's not super obvious when you look at it, but that's what it's doing. const castSubscription = subject.subscribe(subscriber); was the confusing bit I think... because technically, castSubscription === subscriber. :)

Calling subscribe with our own Subscriber will return the same instance of Subscriber.

[cartant]

@benlesh LGTM. I'd approve it, but I can't approve my own PR. 🙂