fix #except and #only with default_scope

[jrom]

Before this fix Model.except(something) wouldn't get rid of 'something'
clauses in the query. Same thing for Model.only().

The two deleted lines were introduced here to "evaluate default scopes at the last possible moment" and that made impossible to remove conditions from the default scope. The behavior after this fix is the same we had before Rails 3.1, which means #except and #only can modify the default_scope.

[jonleighton]

Hmm, this is not really fixing it properly because it will simply mean that a relation after only or except won't have any default scope at all.

[jrom]

Oh shit, you're right. For some reason I had a test that checked that before and I ended up removing this code. I refactored my fix to the simplest solution but it turns out I screwed it so now it just loses the scope.

I can confirm it just loses the default scope:

>> User.count
   (0.5ms)  SELECT COUNT(*) FROM `users` WHERE ((`users`.`deleted` IS NULL OR `users`.`deleted` = 0))
=> 5
>> User.except(:order).count
   (0.4ms)  SELECT COUNT(*) FROM `users` 
=> 5

[jrom]

Added a failing test for this here