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

activerecord/CHANGELOG.md

@@ -1,5 +1,33 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   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
+
+    *Lilibeth De La Cruz*
+
 *   When `#count` is used in conjunction with `#uniq` we perform `count(:distinct => true)`.
     Fix #6865.
 

activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

@@ -40,9 +40,9 @@ def #{attr_name}=(original_time)
                   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

activerecord/test/cases/dirty_test.rb

@@ -79,6 +79,8 @@ def test_time_attributes_changes_with_time_zone
       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