Fix deprecation warnings on Rails 6

takes into account suggestions from @bf4

Author: kylekeesling

lib/action_controller/serialization.rb

@@ -23,7 +23,15 @@ def serialization_scope(scope)
     end
 
     def namespace_for_serializer
-      @namespace_for_serializer ||= self.class.parent unless self.class.parent == Object
+      @namespace_for_serializer ||= namespace_for_class(self.class) unless namespace_for_class(self.class) == Object
+    end
+
+    def namespace_for_class(klass)
+      if Module.method_defined?(:module_parent)
+        klass.module_parent
+      else
+        klass.parent
+      end
     end
 
     def serialization_scope

test/action_controller/namespace_lookup_test.rb

@@ -125,7 +125,12 @@ def namespace_set_by_request_headers
       tests Api::V3::LookupTestController
 
       setup do
-        @test_namespace = self.class.parent
+        @test_namespace =
+          if Module.method_defined?(:module_parent)
+            self.class.module_parent
+          else
+            self.class.parent
+          end
       end
 
       test 'uses request headers to determine the namespace' do