Fix the ._get_db() attribute after a Document.switch_db()

Without this patch, I've got:

```
myobj._get_db()

<bound method TopLevelDocumentMetaclass._get_db of <class 'oneflow.core.models.nonrel.Article'>>

```

I need to `myobj._get_db()()` to get the database.

I felt this like a bug.

regards,