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.

Rakefile

@@ -44,7 +44,32 @@ Rake::TestTask.new do |t|
   t.verbose = true
 end
 
-task default: [:test, :rubocop]
+desc 'Run isolated tests'
+task isolated: ['test:isolated:railtie']
+namespace :test do
+  namespace :isolated do
+    desc 'Run isolated tests for Railtie'
+    task :railtie do
+      dir = File.dirname(__FILE__)
+      file = "#{dir}/test/active_model_serializers/railtie_test_isolated.rb"
+
+      # https://github.com/rails/rails/blob/3d590add45/railties/lib/rails/generators/app_base.rb#L345-L363
+      _bundle_command = Gem.bin_path('bundler', 'bundle')
+      require 'bundler'
+      Bundler.with_clean_env do
+        command = "-w -I#{dir}/lib -I#{dir}/test #{file}"
+        full_command = %("#{Gem.ruby}" #{command})
+        system(full_command) or fail 'Failures' # rubocop:disable Style/AndOr
+      end
+    end
+  end
+end
+
+if ENV['RAILS_VERSION'].to_s > '4.0' && RUBY_ENGINE == 'ruby'
+  task default: [:isolated, :test, :rubocop]
+else
+  task default: [:test, :rubocop]
+end
 
 desc 'CI test task'
 task :ci => [:default]