Possible bug with only and to_json/as_pymongo

Taking this example:

```
>>> class Doc(Document):
...    foo = StringField()
...    bar = StringField()
```
If I want the "bar" field:
```
>>> Doc(foo='foo', bar='bar').save()
>>> Doc.objects.only('bar').to_json()
'[{"bar": "bar"}]'
```
If I want the "id":
```
>>> Doc.objects.only('id').to_json()
'[{"_id": {"$oid": "51cda...bd8a0"}}]'
```
If I want the "id" field and "bar":
```
>>> Doc.objects.only('id', 'bar').to_json()
'[{"bar": "bar"}]'
```
Is this intentional or a bug?