Prevent not-intended loading of `ActionDispatch::IntegrationTest`

After 9d378747 `ActionDispatch::IntegrationTest`
class is loaded and defined in all Rails environments, not only test but also
production. This is not-intended loading of a class which is only used in
test environment.
To prevent not-intended loading, add `ActiveSupport.run_load_hooks` to
`ActionDispatch::IntegrationTest` with `action_dispatch_integration_test` name
and use it in `ActionMailer`.

actionmailer/lib/action_mailer/railtie.rb

@@ -45,9 +45,9 @@ class Railtie < Rails::Railtie # :nodoc:
         register_observers(options.delete(:observers))
 
         options.each { |k,v| send("#{k}=", v) }
-
-        ActionDispatch::IntegrationTest.send :include, ActionMailer::TestCase::ClearTestDeliveries
       end
+
+      ActiveSupport.on_load(:action_dispatch_integration_test) { include ActionMailer::TestCase::ClearTestDeliveries }
     end
 
     initializer "action_mailer.compile_config_methods" do

actionpack/CHANGELOG.md

+*   Add `ActiveSupport.run_load_hooks` to `ActionDispatch::IntegrationTest`
+    with `action_dispatch_integration_test` name.
+
+    *Yuichiro Kaneko*
+
 *   Update default rendering policies when the controller action did
     not explicitly indicate a response.
 

actionpack/lib/action_dispatch/testing/integration.rb

@@ -764,5 +764,7 @@ def document_root_element
     def self.register_encoder(*args)
       Integration::Session::RequestEncoder.register_encoder(*args)
     end
+
+    ActiveSupport.run_load_hooks(:action_dispatch_integration_test, self)
   end
 end