* Rename NullAdapter to SimpleAdapter

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

lib/active_model/serializer/adapter.rb

@@ -0,0 +1,20 @@
+module ActiveModel
+  class Serializer
+    class Adapter
+      extend ActiveSupport::Autoload
+      autoload :SimpleAdapter
+
+      def initialize(serializer)
+        @attributes = serializer.attributes
+      end
+
+      def serializable_hash(options = {})
+        raise NotImplementedError, 'This is abstract method. Should be implemented at concrete adapter.'
+      end
+
+      def to_json(options = {})
+        raise NotImplementedError, 'This is abstract method. Should be implemented at concrete adapter.'
+      end
+    end
+  end
+end

lib/active_model/serializer/adapter/simple_adapter.rb

@@ -1,11 +1,7 @@
 module ActiveModel
   class Serializer
     class Adapter
-      class NullAdapter
-        def initialize(adapter)
-          @attributes = adapter.attributes 
-        end
-
+      class SimpleAdapter < Adapter
         def to_json(options={})
           @attributes.each_with_object({}) do |(attr, value), h|
             h[attr] = value