Define autosave association callbacks when using accepts_nested_attributes_for.

This way we don't define all the validation methods for all associations by
default, but only when needed.

[#3355 state:resolved]

activerecord/lib/active_record/nested_attributes.rb

@@ -250,6 +250,8 @@ def #{association_name}_attributes=(attributes)
                 assign_nested_attributes_for_#{type}_association(:#{association_name}, attributes)
               end
             }, __FILE__, __LINE__
+
+            add_autosave_association_callbacks(reflection)
           else
             raise ArgumentError, "No association found for name `#{association_name}'. Has it been defined yet?"
           end

activerecord/test/models/pirate.rb

@@ -18,7 +18,7 @@ class Pirate < ActiveRecord::Base
   has_many :treasure_estimates, :through => :treasures, :source => :price_estimates
 
   # These both have :autosave enabled because accepts_nested_attributes_for is used on them.
-  has_one :ship, :validate => true
+  has_one :ship
   has_one :non_validated_ship, :class_name => 'Ship'
   has_many :birds
   has_many :birds_with_method_callbacks, :class_name => "Bird",

activerecord/test/models/ship.rb

 class Ship < ActiveRecord::Base
   self.record_timestamps = false
 
-  belongs_to :pirate, :validate => true
+  belongs_to :pirate
   has_many :parts, :class_name => 'ShipPart', :autosave => true
 
   accepts_nested_attributes_for :pirate, :allow_destroy => true, :reject_if => proc { |attributes| attributes.empty? }