Just found a little bug.

if schema like this:
```javascript
	_id: {
		type: Buffer,
		default: function() {
			return (new MongooseBuffer(uuid.v4())).toObject(0x04);
		}
	}
```
the sub_type will be lost.

And I think I really need the spec that Heckmann has mentioned before  >> https://github.com/LearnBoost/mongoose/pull/1000#issuecomment-6912708