`ipl\Orm\Query::assembleSelect()` causes unintended side-effects

Build a query similar to this:

```php
Incident::on(Database::get())
    ->with(['object'])
    ->withColumns([
        'object.id_tags',
        'object.source.name',
        'object.source.type'
    ]);
```

Now `->count()` said query and then iterate it.

You'll notice that the iteration now includes columns you didn't ask for.