Move caching initialization to Railtie

Also
- Add reference to config from ActiveModelSerializers.config
- correctly call super in FragmentCacheTest#setup
- rename test rails app from Foo to ActiveModelSerializers::RailsApplication

lib/active_model/serializer.rb

@@ -152,7 +152,7 @@ module ActiveModel
     # @todo require less code comments. See
     # https://github.com/rails-api/active_model_serializers/pull/1249#issuecomment-146567837
     def self.cache(options = {})
-      self._cache = ActionController::Base.cache_store if Rails.configuration.action_controller.perform_caching
+      self._cache = ActiveModelSerializers.config.cache_store if ActiveModelSerializers.config.perform_caching
       self._cache_key = options.delete(:key)
       self._cache_only = options.delete(:only)
       self._cache_except = options.delete(:except)

lib/active_model/serializer/railtie.rb

@@ -8,6 +8,13 @@ module ActiveModel
       end
     end
 
+    initializer 'active_model_serializers.caching' do
+      ActiveSupport.on_load(:action_controller) do
+        ActiveModelSerializers.config.cache_store     = ActionController::Base.cache_store
+        ActiveModelSerializers.config.perform_caching = Rails.configuration.action_controller.perform_caching
+      end
+    end
+
     initializer 'generators' do |app|
       app.load_generators
       require 'generators/serializer/resource_override'