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

[eturino]

scope_for_slug_generator method on not new records, with History and Scoped enabled, produced invalid SQL statements.

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:

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'

[norman]

Thanks for this, and sorry for taking so long to respond. I've looked into the issue the the real problem is that the scoped and history modules are order-dependent (which they should not be). That is, if you load :scoped before :history if fails, but if you load :history before :scoped then it's fine. There's a similar but simpler fix in #588, I'll merge that today and will push out a new version some time this week (possibly today depending on my day goes).