Fix preparing the configured Action Mailbox ingress in production

When reloading is disabled, after_initialize hooks run after to_prepare hooks have already run for the last time. Since to_prepare hooks always run after application initializers, we don't need need to use an after_initialize hook.

actionmailbox/lib/action_mailbox/engine.rb

@@ -29,10 +29,9 @@ class Engine < Rails::Engine
       end
     end
 
-    initializer "action_mailbox.ingress" do
-      config.after_initialize do |app|
-        if ActionMailbox.ingress = app.config.action_mailbox.ingress.presence
+    initializer "action_mailbox.ingress" do |app|
       config.to_prepare do
+        if ActionMailbox.ingress = app.config.action_mailbox.ingress.presence
           if ingress_controller_class = "ActionMailbox::Ingresses::#{ActionMailbox.ingress.to_s.classify}::InboundEmailsController".safe_constantize
             ingress_controller_class.prepare
           end
@@ -40,5 +39,4 @@ class Engine < Rails::Engine
       end
     end
   end
-  end
 end