Wrap that User in a string (thoughtbot#813)

DEPRECATION WARNING: Passing a class to the  is deprecated and will raise an ArgumentError in Rails 5.2. It eagerloads more classes than necessary and potentially creates circular dependencies. Please pass the class name as a string:  (called from <class:Invitation> at /Users/BenAMorgan/Sites/administrate/spec/generators/dashboard_generator_spec.rb:273)

spec/generators/dashboard_generator_spec.rb

@@ -270,7 +270,7 @@ class Comment < ActiveRecord::Base
           end
           class User < ActiveRecord::Base; end
           class Invitation < ActiveRecord::Base
-            belongs_to :sender, class_name: User
+            belongs_to :sender, class_name: "User"
             belongs_to :recipient, class_name: "User"
           end