when rotate, the width and height of pic gets larger

When I call roatet serval on the same object, the width and height of pic gets larger.
after call rotate about 10 times, the pic will have a quiet obvious black border.

The follwoing code:
```
Jimp.read('https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/koala.jpg', (error, lenna) => {
if (error) throw error;
lenna
.rotate(90)
.rotate(90)
.rotate(90)
.rotate(90)
.rotate(90)
.rotate(90)
.rotate(90)
.getBase64(Jimp.AUTO, (err, base64) => {
this.imageRef.setAttribute('src', base64);
});
});
```

# Expected Behavior

The pic rotate with the original width and height.

# Current Behavior

the pic will have black border ,the width and height of pic will grow evey time.

## Context

- Jimp Version: ^0.8.5
- Operating System: win10
- Node version: v10.16.3