'_id' and/or 'id' properties in subdocuments

Added the possibility to have properties called '_id' or 'id' within subdocuments.
I'm not sure if this was intentionally left out, but I do have a case where I have a document with the following structure and I need to query on `author._id`:

```json
{
	"_id": "",
	"title": "",
	"body": "",
	"author": {
		"_id": "",
		"name": ""
	}
}
```