initial value factory overload for scan?

[davidmoten]

I was just using Observable.scan() and it has a method with an initial value but no method with an intial value factory like:

public final <R> Observable<R> scan(Func0<R> initialValue, Func2<R, ? super T, R> accumulator); 

I can work around this with defer but are people happy if I submit a PR with a factory parameter (and perhaps for the reduce method as well)?

[zsxwing]

See #3667

[akarnokd]

Closing as duplicate to #3667.