Code gets removed even with `afterOptimizations` to `true`

I'm using the plugin like this:
```js
new WrapperPlugin({
 header: (fileName, { hash }) => {
    return fileName === 'serviceWorker.bundle.js' ? `/* hash: ${hash} */` : ''
  },
  afterOptimizations: true
})
```
When running in development mode, the comment is there, but when running in production mode, the comment is not there.

I think this might be because of #11.