Move use of serializers cache out of Utils

lib/active_model/serializable.rb

@@ -33,7 +33,11 @@ module ActiveModel
     end
 
     def namespace
-      get_namespace && Utils._const_get(get_namespace)
+      if module_name = get_namespace
+        Serializer.serializers_cache.fetch_or_store(module_name) do
+          Utils._const_get(module_name)
+        end
+      end
     end
 
     def embedded_in_root_associations

lib/active_model/serializable/utils.rb

@@ -4,7 +4,6 @@ module ActiveModel
       extend self
 
       def _const_get(const)
-        Serializer.serializers_cache.fetch_or_store(const) do
         begin
           method = RUBY_VERSION >= '2.0' ? :const_get : :qualified_const_get
           Object.send method, const
@@ -15,4 +14,3 @@ module ActiveModel
     end
   end
 end
-end

lib/active_model/serializer.rb

@@ -66,7 +66,10 @@ end
             ArraySerializer
           end
         else
-          _const_get build_serializer_class(resource, options)
+          klass_name = build_serializer_class(resource, options)
+          Serializer.serializers_cache.fetch_or_store(klass_name) do
+            _const_get(klass_name)
+          end
         end
       end