fix(Slugged+History+Scoped): fix SQL errors when using the 3 plugins together

`scope_for_slug_generator` method on not new records, with History and Scoped enabled, produced invalid SQL statements.

```sql
SELECT "restaurants".* FROM "restaurants"  
  WHERE "restaurants"."city_id" = 1 
  AND ("restaurants"."id" != 1) 
  AND (sluggable_id <> 1) 
  AND "restaurants"."scope" = 'city_id:1'
```

SCOPE is part of the Slug model, not the Restaurant model:

```sql
SELECT "restaurants".* FROM "restaurants"  
  WHERE "restaurants"."city_id" = 1 
  AND ("restaurants"."id" != 1) 
  AND (sluggable_id <> 1) 
  AND "friendly_id_slugs"."scope" = 'city_id:1'
```