Only call railtie when Rails is defined; assume controller loaded

Isolated Testing

- Rake test inspired by https://github.com/rails/rails/blob/v5.0.0.beta1/activejob/Rakefile
- Isolated unit inspired by
  - https://github.com/rails/rails/blob/v5.0.0.beta1/railties/test/isolation/abstract_unit.rb
  - https://github.com/rails/rails/blob/v5.0.0.beta1/activemodel/test/cases/railtie_test.rb

Misc

- Turns out `mattr_accessor(:logger) {
  ActiveSupport::TaggedLogging.new(ActiveSupport::Logger.new(STDOUT)) }`
  was always nil until the Railtie was loaded, since mattr_accessor
  block defaults don't really work on modules, but on the classes that
  include them.
- Commented on important on Rails being required first for caching to
  work.
- In isolated tests, `active_support/core_ext/object/with_options` is required.

lib/active_model_serializers.rb

@@ -1,7 +1,6 @@
 require 'active_model'
 require 'active_support'
-require 'action_controller'
-require 'action_controller/railtie'
+require 'active_support/core_ext/object/with_options'
 module ActiveModelSerializers
   extend ActiveSupport::Autoload
   autoload :Model
@@ -10,7 +9,8 @@ module ActiveModelSerializers
   autoload :Logging
   autoload :Test
 
-  mattr_accessor(:logger) { ActiveSupport::TaggedLogging.new(ActiveSupport::Logger.new(STDOUT)) }
+  class << self; attr_accessor :logger; end
+  self.logger = ActiveSupport::TaggedLogging.new(ActiveSupport::Logger.new(STDOUT))
 
   def self.config
     ActiveModel::Serializer.config
@@ -18,7 +18,6 @@ module ActiveModelSerializers
 
   require 'active_model/serializer/version'
   require 'active_model/serializer'
-  require 'active_model_serializers/railtie'
   require 'active_model/serializable_resource'
-  require 'action_controller/serialization'
+  require 'active_model_serializers/railtie' if defined?(::Rails)
 end

lib/active_model_serializers/railtie.rb

@@ -1,4 +1,7 @@
 require 'rails/railtie'
+require 'action_controller'
+require 'action_controller/railtie'
+require 'action_controller/serialization'
 
 module ActiveModelSerializers
   class Railtie < Rails::Railtie
@@ -7,10 +10,8 @@ module ActiveModelSerializers
     end
 
     initializer 'active_model_serializers.action_controller' do
-      ActiveSupport.on_load(:action_controller) do
-        ActiveSupport.run_load_hooks(:active_model_serializers, ActiveModelSerializers)
-        include ::ActionController::Serialization
-      end
+      ActiveSupport.run_load_hooks(:active_model_serializers, ActiveModelSerializers)
+      ActionController::Base.send(:include, ::ActionController::Serialization)
     end
 
     initializer 'active_model_serializers.logger' do
@@ -19,11 +20,19 @@ module ActiveModelSerializers
       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
+    # To be useful, this hook must run after Rails has initialized,
+    # BUT before any serializers are loaded.
+    # Otherwise, the call to 'cache' won't find `cache_store` or `perform_caching`
+    # defined, and serializer's `_cache_store` will be nil.
+    # IF the load order cannot be changed, then in each serializer that that defines a `cache`,
+    # manually specify e.g. `PostSerializer._cache_store = Rails.cache` any time
+    # before the serializer is used.  (Even though `ActiveModel::Serializer._cache_store` is
+    # inheritable, we don't want to set it on `ActiveModel::Serializer` directly unless
+    # we want *every* serializer to be considered cacheable, regardless of specifying
+    # `cache # some options` in a serializer or not.
+    initializer 'active_model_serializers.caching' => :after_initialize do
+      ActiveModelSerializers.config.cache_store     = ActionController::Base.cache_store
+      ActiveModelSerializers.config.perform_caching = Rails.configuration.action_controller.perform_caching
     end
 
     generators do