Eager autoload Preloader classes

Without eager autoloading, these would be autoloaded
only when #preloader_for is called, which is too late
in threaded applications.

activerecord/lib/active_record/associations/preloader.rb

@@ -30,6 +30,9 @@ module Associations
     # option references an association's column), it will fallback to the table
     # join strategy.
     class Preloader #:nodoc:
+      extend ActiveSupport::Autoload
+
+      eager_autoload do
         autoload :Association,           'active_record/associations/preloader/association'
         autoload :SingularAssociation,   'active_record/associations/preloader/singular_association'
         autoload :CollectionAssociation, 'active_record/associations/preloader/collection_association'
@@ -41,6 +44,7 @@ class Preloader #:nodoc:
         autoload :HasOneThrough,       'active_record/associations/preloader/has_one_through'
         autoload :HasAndBelongsToMany, 'active_record/associations/preloader/has_and_belongs_to_many'
         autoload :BelongsTo,           'active_record/associations/preloader/belongs_to'
+      end
 
       attr_reader :records, :associations, :preload_scope, :model