fix default include in default scope fails findById

If there is an include option in default scope, `findById()` will complaint:

> Relation "things" is not defined for Thing model

I think it's because the default scope is applied twice, the first is in `byIdQuery()` which is called by `findById()`, the second is in `find()`. This will cause `mergeQuery()` to nest the include option. 

Signed-off-by: Clark Wang <clark.wangs@gmail.com>