in_scope?

The patch adds new scope functionality that I've found very useful in my project. It simply adds new instance method called in_scope? to each defined named_scope. I use it in several cases, i.e.

class User
  named_scope :destroyable, :conditions => "..." # simple association check
  before_destroy :in_destroyable?
end

user = User.destroyable.first

user.in_destroyable? # the same as User.destroyable.include?(user)
#=> true

Firstly I used :destroyable? method (without "in_") but it may collide with some boolean attributes like "active", etc. Implementation is very simple and it uses ordinary :include? method to check the inclusion. Probably it can be optimised to do the inclusion check in SQL, but for me it's really enough.