[model - belongsTo] Changing the comparison of id and fk of belongsTo function.

I've tried to use belongsTo function with mongoDb Database.

```coffeescript
Object.belongsTo Category, as: 'categorized',  foreignKey: 'categoryId'
@object.categorized (category_id, category) =>
    @category = category || []
```

When I call function, he return a "Permission denied" response.

So I've try to debug and I've found that:

```javascript
typeof inst.id === object 
typeof this[fk] === string
```

I therefore think that comparison type is not usefull and I would like to suggest a fix.

Thanks.