[Bug] `reduce` passes incorrect `index` when `initialValue` is excluded

**Describe the bug**
The `reduce` implementation passes the incorrect `index` value when `initialValue` is excluded; it should start at `1`, instead of `0`.

**To Reproduce**
```js
Iterator.from([1,2,3]).reduce((s, v, i) => i)
// -> 1
```

**Expected behavior**
Should return `2`.

**Screenshots**
N/A

**Additional context**
N/A