Synchronous emit in flush method of debounce produces incorrect result

If you will try to do synchronous emit in debounced observable incorrect sequence of events will be produced

i.e.

const a = Kefir.pool();
const b = a.debounce(100).spy();
a.plug(b.filter((val) => val === 1).map(() => 2));
a.plug(Kefir.constant(1));
a.observe();

Expected output

100ms [pool.debounce] <value> 1
200ms [pool.debounce] <value> 2

Actual output

100ms [pool.debounce] <value> 1
200ms [pool.debounce] <value> null

[mAAdhaTTah]

@bpinto Done. Let me get a release out this weekend.

[bpinto]

Hey guys, @betalb opened a PR fixing this, do you think we could merge their PR?