Allow overriding the adapter with render option

Make it easy to use multiple adapters in an app.

use "adapter: false" to not use ams

make a test override config.adapter

test/adapter_test.rb

@@ -18,6 +18,26 @@ module ActiveModel
       def test_serializer
         assert_equal @serializer, @adapter.serializer
       end
+
+      def test_adapter_class_for_known_adapter
+        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_api)
+        assert_equal ActiveModel::Serializer::Adapter::JsonApi, klass
+      end
+
+      def test_adapter_class_for_unknown_adapter
+        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_simple)
+        assert_nil klass
+      end
+
+      def test_create_adapter
+        adapter = ActiveModel::Serializer::Adapter.create(@serializer)
+        assert_equal ActiveModel::Serializer::Adapter::Json, adapter.class
+      end
+
+      def test_create_adapter_with_override
+        adapter = ActiveModel::Serializer::Adapter.create(@serializer, { adapter: :json_api})
+        assert_equal ActiveModel::Serializer::Adapter::JsonApi, adapter.class
+      end
     end
   end
 end