findIn not work with relation

BookModel relation definition:
```
author_id: this.number(0),
author: this.belongsTo(AuthorModel, 'author_id'),
```
author is null when using findIn like:
`BookModel.query().with('author').findIn([1, 2])`
and works well with whereIdIn:
`BookModel.query().with('author').whereIdIn([1, 2]).get()`