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

(cherry picked from commit bc982cbcb34129ea2cfe8aa1f8e0b40e444e68db)

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

Backport of #9073
Fixes #8898

activerecord/CHANGELOG.md

@@ -1,5 +1,36 @@
 ## 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
+
+    Backport of #9073
+    Fixes #8898
+
+    *Lilibeth De La Cruz*
+
 *   Fix counter cache columns not updated when replacing `has_many :through`
     associations.
     Backport #8400.

activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

@@ -43,9 +43,9 @@ 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

activerecord/test/cases/dirty_test.rb

@@ -76,6 +76,8 @@ 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