ReactiveX / rxjs / 3704

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fix(reduce/scan): both scan/reduce operators now accepts `undefined` itself as a valid seed (#2050)

* fix(scan): scan operator now accepts `undefined` itself as a valid seed value

Array#reduce supports `undefined` as a valid seed value, so it seems
natural that we would too for scan

```js
of(1, 2, 3).scan((acc, x) => acc + ' ' + x, undefined);
// "undefined 1"
// "undefined 1 2"
// "undefined 1 2 3"
```

fixes #2047

* fix(reduce): reduce operator now accepts `undefined` itself as a valid seed value

Array#reduce supports `undefined` as a valid seed value, so it seems
natural that we would too.

```js
of(1, 2, 3).reduce((acc, x) => acc + ' ' + x, undefined);
// "undefined 1 2 3"
```

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