[Feature request] Resources with strict role

[gabriel-curtino]

May worth to complement the strict mode for resources too, like this:

class Forum < ActiveRecord::Base
  resourcify strict: true
end

@user = User.first
@user.add_role(:admin, Forum.first) #=> ok
@user.add_role(:admin, Forum) #=> Exception?

This can work, in fact, as a security enforcement allowing only role assignment over instances.