* Rename NullAdapter to SimpleAdapter

* Introduce abstract Adapter class
* Organaze test structure to match convemtions

test/adapter_test.rb

@@ -0,0 +1,25 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class AdapterTest < Minitest::Test
+      def setup
+        profile = Profile.new
+        serializer = ProfileSerializer.new(profile)
+        @adapter = ActiveModel::Serializer::Adapter.new(serializer)
+      end
+
+      def test_serializable_hash_is_abstract_method
+        assert_raises(NotImplementedError) do
+          @adapter.serializable_hash(only: [:name])
+        end
+      end
+
+      def test_serializable_hash_is_abstract_method
+        assert_raises(NotImplementedError) do
+          @adapter.to_json(only: [:name])
+        end
+      end
+    end
+  end
+end