Full STI class name in associations

When using namespaced models together with has_many and :joins in find the demodulized class name is used instead of the full class name even so the "store_full_sti_class" option is set to "true".

Example:

module Namespaced
  class Firm < ::Company
    has_many :clients, :class_name => 'Namespaced::Client'
  end

  class Client < ::Company
  end
end

Namespaced::Firm.find(:all, {
  :select => "#{Namespaced::Firm.table_name}.id, COUNT(#{Namespaced::Client.table_name}.id)",
  :joins => :clients,
  :group => "#{Namespaced::Firm.table_name}.id"
})

will use type = 'Client' instead of type = 'Namespaced::Client'

Patch with tests attached.