Setting Background Color of Image not working

# Expected Behavior

Color the Image Background to specified color

# Current Behavior

Image Background doesn't change color

# Failure Information (for bugs)

No Error

## Steps to Reproduce

1. Read Image
2. Set Background color
3. Write to File

```
return Jimp.read('app/background.png')
.then(image => {
image.background(0x0012faff);
})
.then(image => {
return image.writeAsync(`app/new-${(new Date()/1000)}.png`);
})
```

## Context

- Jimp Version: 0.6.0
- Operating System: Mac Mojave
- Node version: 7.0.0

## Failure Logs