fix(debounceTime): align value emit behavior as same as RxJS4

[kwonoj]

due to current null check, debounceTime does not allow value
to be undefined. This change updates behavior to align behavior
as same as RxJS4 does.

closes #1081

[kwonoj]

I think this change's legit, but will wait to see if any other opinion / suggestions around before checkin.

[mattpodwysocki]

@kwonoj why are you checking for null/undefined, when you should have a separate flag instead for hasValue just as we did in RxJS v4? After all, null is also a valid value to send along. Take a look at the current v4 implementation: https://github.com/Reactive-Extensions/RxJS/blob/master/src/modular/observable/debounce.js

[kwonoj]

Thanks for suggestion, it's quite valid point. Current implementation used null instead of having hasValue to check it has value or not, so as pointed out it won't be able to accept it as value. I'll update PR to reflect behavior correctly.

[staltz]

Ah, this PR exists... nice.

[kwonoj]

PR updated to introduce hasValue instead of current implementation uses null to check if value has been set or not, to avoid some of values are not being accepted correctly.

[kwonoj]

Merged with 5ee11e0.

[lock]

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