Unexpected anonymousField

```python
ns = pjs.ObjectBuilder({
    "id": "Example",
    "type": "object",
    "properties": {
        "prop": {
            "title": "prop",
            "type": "array",
            "items": { "type": "string" }
        }
    }
}).build_classes()
print(dir(ns))
```

Output:
```
['Example', 'Prop<anonymousField>']
```

Why is `Prop` an `anonymousField`? Is this expected?
