Support for {root: false}

First of all, thanks for an awesome project.

---

When I render with plain active model serializers with

```
render :json => @events, root: false
```

It produces an array of objects.

However when I put postgres_ext-serializers in place, it returns 

```
{"events": [same array as before]}
```

 In other words, the option of removing the root does not seem to be working.
