restore `delivery_method` after test

Currently use two variables to restore `delivery_method`, but the order
of restoring the values is incorrect and does not restore is correctly.

actionmailer/test/parameterized_test.rb

@@ -9,10 +9,9 @@ class ParameterizedTest < ActiveSupport::TestCase
     @previous_logger = ActiveJob::Base.logger
     ActiveJob::Base.logger = Logger.new(nil)
 
-    @original_delivery_method = ActionMailer::Base.delivery_method
+    @previous_delivery_method = ActionMailer::Base.delivery_method
     ActionMailer::Base.delivery_method = :test
 
-    @previous_delivery_method = ActionMailer::Base.delivery_method
     @previous_deliver_later_queue_name = ActionMailer::Base.deliver_later_queue_name
     ActionMailer::Base.deliver_later_queue_name = :test_queue
     ActionMailer::Base.delivery_method = :test
@@ -24,8 +23,6 @@ class ParameterizedTest < ActiveSupport::TestCase
     ActiveJob::Base.logger = @previous_logger
     ParamsMailer.deliveries.clear
 
-    ActionMailer::Base.delivery_method = @original_delivery_method
-
     ActionMailer::Base.delivery_method = @previous_delivery_method
     ActionMailer::Base.deliver_later_queue_name = @previous_deliver_later_queue_name
   end