…ed 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 ReactiveX#2047

…d 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"
```