Only call save on belongs_to associations if the record has changed or any...

Only call save on belongs_to associations if the record has changed or any nested associations have changed (resolves #3353)
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

activerecord/lib/active_record/autosave_association.rb

@@ -363,7 +363,7 @@ def save_belongs_to_association(reflection)
         if autosave && association.marked_for_destruction?
           association.destroy
         elsif autosave != false
-          saved = association.save(:validate => !autosave) if association.new_record? || autosave
+          saved = association.save(:validate => !autosave) if association.new_record? || (autosave && association.changed_for_autosave?)
 
           if association.updated?
             association_id = association.send(reflection.options[:primary_key] || :id)

activerecord/test/cases/autosave_association_test.rb

@@ -667,10 +667,21 @@ def save(*args)
       end
     end
 
+    @ship.pirate.catchphrase = "Changed Catchphrase"
+
     assert_raise(RuntimeError) { assert !@ship.save }
     assert_not_nil @ship.reload.pirate
   end
 
+  def test_should_save_changed_child_objects_if_parent_is_saved
+    @pirate = @ship.create_pirate(:catchphrase => "Don' botharrr talkin' like one, savvy?")
+    @parrot = @pirate.parrots.create!(:name => 'Posideons Killer')
+    @parrot.name = "NewName"
+    @ship.save
+
+    assert_equal 'NewName', @parrot.reload.name
+  end
+
   # has_many & has_and_belongs_to
   %w{ parrots birds }.each do |association_name|
     define_method("test_should_destroy_#{association_name}_as_part_of_the_save_transaction_if_they_were_marked_for_destroyal") do