Fix AuthenticationGeneratorTest

Reassign the Railtie into the right constant.

Author: byroot

railties/test/generators/authentication_generator_test.rb

@@ -155,7 +155,7 @@ def test_authentication_generator_without_action_mailer
       assert_no_match(/rate_limit/, content)
     end
   ensure
-    ActionCable.const_set(:Railtie, old_value)
+    ActionMailer.const_set(:Railtie, old_value)
   end
 
   private