About signed zero

Is this correct?

```
var zero = new BN(0);
zero.toString();
> '0'
zero.subn(1).addn(1).toString();
> '-0'
```

I expect the following behavior.

```
var zero = new BN(0);
zero.toString();
> '0'
zero.subn(1).addn(1).toString();
> '0'
```