Correctly check whether key is defined in configuration

It can not check correctly with `defined?`

```ruby
irb(main):001:0> Rails.application.config.active_record
=> {:maintain_test_schema=>true, :belongs_to_required_by_default=>true}
irb(main):002:0> defined?(Rails.application.config.active_record)
=> nil
```

Follow up to #28469

1 files changed, 3 insertions, 3 deletions

diff --git a/railties/lib/rails/application/configuration.rb b/railties/lib/rails/application/configuration.rb
index 027207bbb2..819e801740 100644
--- a/railties/lib/rails/application/configuration.rb
+++ b/railties/lib/rails/application/configuration.rb
@@ -58,14 +58,14 @@ module Rails
       def load_defaults(target_version)
         case target_version.to_s
         when "5.0"
-          if defined?(action_controller)
+          if respond_to?(:action_controller)
             action_controller.per_form_csrf_tokens = true
             action_controller.forgery_protection_origin_check = true
           end
 
           ActiveSupport.to_time_preserves_timezone = true
 
-          if defined?(active_record)
+          if respond_to?(:active_record)
             active_record.belongs_to_required_by_default = true
           end
 
@@ -74,7 +74,7 @@ module Rails
         when "5.1"
           load_defaults "5.0"
 
-          if defined?(assets)
+          if respond_to?(:assets)
             assets.unknown_asset_fallback = false
           end