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, 2 deletions

diff --git a/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb b/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
index 7701001da9..e3c5dc4823 100644
--- a/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
+++ b/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
@@ -40,9 +40,9 @@ module ActiveRecord
                   time = time.is_a?(String) ? Time.zone.parse(time) : time.to_time rescue time
                 end
                 time = time.in_time_zone rescue nil if time
-                changed = read_attribute(:#{attr_name}) != time
+                previous_time = attribute_changed?("#{attr_name}") ? changed_attributes["#{attr_name}"] : read_attribute(:#{attr_name})
                 write_attribute(:#{attr_name}, original_time)
-                #{attr_name}_will_change! if changed
+                #{attr_name}_will_change! if previous_time != time
                 @attributes_cache["#{attr_name}"] = time
               end
             EOV