Add failing test case for #249

[mAAdhaTTah]

Subscribing to a circular dep should cause everything to
deactivate but doesn't.

I don't have a solution to this yet, but if anyone has ideas why this might be happening, we at least have a test case we can use to verify the solution.

[rpominov]

This seems like something impossible to fix without some kind of global supervision that would analyze all observables and detect loops. I mean from the point of view of individual observables there is no way to notice a loop and deactivate itself, i.e. notice that the only subscriber that left is effectively the same observable.

[mAAdhaTTah]

notice that the only subscriber that left is effectively the same observable.

I'd probably have to dig into the internals a bit more, but if the only subscriber isn't active, we don't necessarily need to detect a loop, do we?

[rpominov]

but if the only subscriber isn't active

A subscriber isn't an observable it's just a callback. The callback is added by an observable, but we have access only to the callback. But even if we had access to the observable it's in fact active because it also have a subscriber (basically from the first observable, via another intermediate observable).

[bpinto]

If this isn't a bug, should we consider closing this issue?