When calling populate inside a pre save hook a copy of the referenced document is saved inline

This just bite me.

What I usually get when saving

```js
{
    _id: ObjectId(...),
    _creator: ObjectId(...)
}
```

What I get when calling `populate` inside a pre save hook

```js
{
    _id: ObjectId(...),
    _creator: {
        /*All properties of the User are now here. No reference.*/
    }
}
```

I didn't actually add failing tests. But adding a `populate` call makes some of the ref tests fail.