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

lib/active_model/serializer.rb

@@ -90,8 +90,7 @@ module ActiveModel
     def self.adapter
       adapter_class = case config.adapter
       when Symbol
-        class_name = "ActiveModel::Serializer::Adapter::#{config.adapter.to_s.classify}"
-        class_name.safe_constantize
+        ActiveModel::Serializer::Adapter.adapter_class(config.adapter)
       when Class
         config.adapter
       end

lib/active_model/serializer/adapter.rb

@@ -20,6 +20,16 @@ module ActiveModel
       def as_json(options = {})
         serializable_hash(options)
       end
+
+      def self.create(resource, options = {})
+        override = options.delete(:adapter)
+        klass = override ? adapter_class(override) : ActiveModel::Serializer.adapter
+        klass.new(resource, options)
+      end
+
+      def self.adapter_class(adapter)
+        "ActiveModel::Serializer::Adapter::#{adapter.to_s.classify}".safe_constantize
+      end
     end
   end
 end