Date Format: fix gregorian BC era (vs. negative date years)

The following negative year is incorrectly formatted using negative values. Eras aren't being handled correctly.

```js
Globalize('en').formatDate(new Date(-1,1,1), {skeleton: 'yMd'})
// > '2/1/-1'
```

Parsing is working fine.

```js
Globalize('en').parseDate('5 BC', {skeleton: 'Gy'})
// > -000004-01-01T02:00:00.000Z
Globalize('en').parseDate('1 BC', {skeleton: 'Gy'})
// > 0000-01-01T02:00:00.000Z
Globalize('en').parseDate('1 BC', {skeleton: 'Gy'})
// > 0000-01-01T02:00:00.000Z
Globalize('en').parseDate('1 AD', {skeleton: 'Gy'})
// > 0001-01-01T02:00:00.000Z
```