fix(SafeSubscriber): Propagate disposal to terminal Subscriber

[trxcllnt]

Description:
Ensure the internal SafeSubscriber is disposed when an Observable is subscribed with a Subscriber or Subscriber-like object.

Related issue (if exists):
#2675

❤️

[benlesh]

Yeesh. It's odd that we missed that for so long.

[cartant]

Sorry for my tardiness in reviewing this. I'll do it tomorrow. It LGTM, but there is one situation with interop observables I want to look into.

[cartant]

LGTM. Checked and there's no relation to the interop business I mentioned in the comment I made.

[benlesh]

There are some concerns here around Subject and Subjects originating from other copies of RxJS (in Node).

[benlesh]

Marking as changes requested, just so I don't accidentally merge this until we're sure there are no bad implications with Subjects and "interop Subjects", which have unsubscribe methods on them.

[trxcllnt]

I think foreign Subjects should be caught by these lines here, and not be wrapped in a Subscriber:

rxjs/src/internal/util/toSubscriber.ts

Lines 16 to 18 in 5cf26e7

	if ((nextOrObserver as any)[rxSubscriberSymbol]) {
	return (nextOrObserver as any)[rxSubscriberSymbol]();
	}

[cartant]

Blocking this because of #5472

[trxcllnt]

I think this was superseded by #5472. Closing.