Using customHash has returned to prepending multiple times again

(note: closed https://github.com/rickharrison/broccoli-asset-rewrite/issues/43 in favor of this issue)

Updated to latest and assets are now getting "assets" three times when using customHash and prepend.

in:
``
out:
``

```
var fingerprintOptions = {
enabled: true,
extensions: ['js', 'css', 'png', 'jpg', 'gif', 'map', 'svg'],
generateAssetMap: true,
prepend: 'http://localhost:4200/',
customHash: null,
generateAssetMap: false
};
```

Looks specific to customHash again (removing that fixes again) and may be related to https://github.com/rickharrison/broccoli-asset-rev/issues/87