Copy config.active_record.* again when ActiveRecord::Base is loaded

Otherwise initializer files with `Rails.application.active_record.*`
might break.

activerecord/lib/active_record/core.rb

@@ -341,6 +341,10 @@ module ActiveRecord
         ActiveRecord.default_timezone
       end
 
+      def maintain_test_schema # :nodoc:
+        ActiveRecord.maintain_test_schema
+      end
+
       def reading_role # :nodoc:
         ActiveSupport::Deprecation.warn(<<~MSG)
           ActiveRecord::Base.reading_role is deprecated and will be removed in Rails 7.0.

activerecord/lib/active_record/migration.rb

@@ -651,7 +651,7 @@ module ActiveRecord
       end
 
       def maintain_test_schema! #:nodoc:
-        if ActiveRecord.maintain_test_schema
+        if ActiveRecord::Base.maintain_test_schema
           suppress_messages { load_schema_if_pending! }
         end
       end

activerecord/lib/active_record/railtie.rb

@@ -224,11 +224,18 @@ To keep using the current cache store, you can turn off cache versioning entirel
         configs.each do |k, v|
           next if k == :encryption
           setter = "#{k}="
-          next if ActiveRecord.respond_to?(setter)
+          # Some existing initializers might rely on Active Record configuration
+          # being copied from the config object to their actual destination when
+          # `ActiveRecord::Base` is loaded.
+          # So to preserve backward compatibility we copy the config a second time.
+          if ActiveRecord.respond_to?(setter)
+            ActiveRecord.send(setter, v)
+          else
             send(setter, v)
           end
         end
       end
+    end
 
     # This sets the database configuration from Configuration#database_configuration
     # and then establishes the connection.