rgba(false) returns garbled png

# Expected Behavior

The alpha channel to be removed from my png without garbling it.

# Current Behavior

It returns a garbled image, sans alpha channel though!

# Failure Information (for bugs)

## Steps to Reproduce

```
const Jimp = require("jimp");

let src = "icon.png";

Jimp.read(src, (err, image) => {
if (err) {
console.log(err);
return;
}
image.rgba(false).write("result.png");
});
```

## Context

- Jimp Version: 0.16.1
- Operating System: Ubuntu 18.04 & macOS 10.15.7
- Node version: 12.7.0