Automatically add accessible_by to Mongoid Documents to match CanCan behavior for ActiveRecord and DataMapper.

Previously, CanCan::ModelAdditions had to be included in each and every Mongoid document separately. Also removed manual include of CanCan::ModelAdditions from Mongoid documents in Mongoid adapter specs.

lib/cancan/model_adapters/mongoid_adapter.rb

@@ -51,3 +51,8 @@ module CanCan
     alias_method :matches_conditions_hash?, :matches_conditions_hash_with_mongoid_subject?
   end
 end
+
+# simplest way to add `accessible_by` to all Mongoid Documents
+module Mongoid::Document::ClassMethods
+  include CanCan::ModelAdditions::ClassMethods
+end