Fix handling of dirty time zone aware attributes

Previously, when `time_zone_aware_attributes` were enabled, after
changing a datetime or timestamp attribute and then changing it back
to the original value, `changed_attributes` still tracked the
attribute as changed. This caused `[attribute]_changed?` and
`changed?` methods to return true incorrectly.

Example:

    in_time_zone 'Paris' do
      order = Order.new
      original_time = Time.local(2012, 10, 10)
      order.shipped_at = original_time
      order.save
      order.changed? # => false

      # changing value
      order.shipped_at = Time.local(2013, 1, 1)
      order.changed? # => true

      # reverting to original value
      order.shipped_at = original_time
      order.changed? # => false, used to return true
    end

1 files changed, 2 insertions, 0 deletions

diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index 4ac82f6880..c7d2ba6073 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -79,6 +79,8 @@ class DirtyTest < ActiveRecord::TestCase
       assert pirate.created_on_changed?
       assert_kind_of ActiveSupport::TimeWithZone, pirate.created_on_was
       assert_equal old_created_on, pirate.created_on_was
+      pirate.created_on = old_created_on
+      assert !pirate.created_on_changed?
     end
   end