find_one() parses ObjectId() if auth_username_field='_id'

According to [this comment](https://github.com/nicolaiarocci/eve/issues/46#issuecomment-16785003) you're supposed to be able to set `auth_username_field` to `ID_FIELD`.

However, this bypasses the conversion [here](https://github.com/nicolaiarocci/eve/blob/develop/eve/io/mongo/mongo.py#L136) where `ID_FIELD` is converted to an `ObjectId`:

    try:
        if config.ID_FIELD in lookup:
            lookup[ID_FIELD] = ObjectId(lookup[ID_FIELD])
    except:
        pass

Running `_datasource_ex()` before the above fixes this.