make sure to delay publishing OnCompleted() on all those methods that use intermediary IObservable

[atsushieno]

I noticed that there are very likely many methods that need to be fixed for "too early OnCompleted() publication" due to use of some intermediary IObservable.

This one is an example fix I made today, which would describe what I mean:
10d889b#L1L1745

We should wait for all relevant IObservables before triggering OnCompleted() just by the source OnCompleted().