Make cache_store reference explicit

This avoids an issue when the base class for application's controllers inherit from `ActionController::API` instead of `ActionController::Base`.

Author: ncuesta

lib/active_model_serializers/railtie.rb

@@ -28,7 +28,7 @@ class Railtie < Rails::Railtie
       ActiveModelSerializers.config.perform_caching = Rails.configuration.action_controller.perform_caching
       # We want this hook to run after the config has been set, even if ActionController has already loaded.
       ActiveSupport.on_load(:action_controller) do
-        ActiveModelSerializers.config.cache_store = cache_store
+        ActiveModelSerializers.config.cache_store = ActionController::Base.cache_store
       end
     end