Implement a NullAdapter.

This adapter basically doesn't do anything, and just serializes
the attributes into plain old JSON.

lib/active_model/serializer/adapter/null_adapter.rb

@@ -0,0 +1,17 @@
+module ActiveModel
+  class Serializer
+    class Adapter
+      class NullAdapter
+        def initialize(serializer)
+          @serializer = serializer
+        end
+
+        def to_json
+          @serializer.attributes.each_with_object({}) do |(attr, value), h|
+            h[attr] = value
+          end.to_json
+        end
+      end
+    end
+  end
+end