fix(expand): terminate recursive call when destination completes

[kwonoj]

relates to #766

Partially resolves expand operator behavior by adding recursive call terminating condition. Still inifnite expand causes stack overflow though.

Feeling this might not be best way to solve issue, any suggestions would be gladly welcome.

[staltz]

@kwonoj this PR might make sense to merge together with #788. With both PRs, the infinite expand of ScalarObservables causes stack overflow eventually, but at a very large number, 579096.

If I add a small delay (hence converting the synchronous ScalarObservable to a normal async Observable), like .expand(function (x) { return Rx.Observable.of(42 + x).delay(1); }), then the infinite expand can easily pass larger numbers like 1000000 and keep on going indefinitely.

So, I don't think very long living synchronous expands is something we need to support. This PR and PR #788 should be enough.

[kwonoj]

I agree this PR need to be merged with #788 to correct its behaviors.

Supporting infinite expand is somewhat debatable, I'm thinking this is similar cases to #651 . In general, with current design operators have weakness to repeat its behavior with sufficiently large number (depends on system, runtime). It would be better to have mechanism instead of recursive calling.

[staltz]

Mmm, yes, agreed.

[kwonoj]

Merged with 3b8cf94.