Support injecting helpers into ActionController::API (#133)

Rails API apps use the lighter alternative to ActionController::Base.
This adds support for injecting these helpers into either action
controller base class that the consuming application happens to inherit
from.

lib/sorcery/engine.rb

@@ -8,9 +8,11 @@ class Engine < Rails::Engine
     config.sorcery = ::Sorcery::Controller::Config
 
     initializer 'extend Controller with sorcery' do
-      ActionController::Base.send(:include, Sorcery::Controller)
-      ActionController::Base.helper_method :current_user
-      ActionController::Base.helper_method :logged_in?
+      ActiveSupport.on_load('action_controller') do
+        send(:include, Sorcery::Controller)
+        helper_method :current_user
+        helper_method :logged_in?
+      end
     end
   end
 end