Enable Rails 5.1 defaults in dummy app

This enables the belongs_to_required_by_default option.
Add "optional: true" where necessary to fix spec failures caused by this.
This will enable users of PaperTrail, who also use the Rails 5.1 defaults,
to not see exceptions caused by "item" being required by VersionConcern, such as when using soft deletion.

lib/paper_trail/version_concern.rb

@@ -12,7 +12,11 @@ module VersionConcern
     extend ::ActiveSupport::Concern
 
     included do
-      belongs_to :item, polymorphic: true
+      if ActiveRecord.gem_version >= Gem::Version.new('5.0')
+        belongs_to :item, polymorphic: true, optional: true
+      else
+        belongs_to :item, polymorphic: true
+      end
 
       # Since the test suite has test coverage for this, we want to declare
       # the association when the test suite is running. This makes it pass when

spec/dummy_app/app/models/fluxor.rb

@@ -1,3 +1,7 @@
 class Fluxor < ActiveRecord::Base
-  belongs_to :widget
+  if ActiveRecord.gem_version >= Gem::Version.new('5.0')
+    belongs_to :widget, optional: true
+  else
+    belongs_to :widget
+  end
 end

spec/dummy_app/app/models/person.rb

@@ -1,7 +1,13 @@
 class Person < ActiveRecord::Base
   has_many :authorships, foreign_key: :author_id, dependent: :destroy
   has_many :books, through: :authorships
-  belongs_to :mentor, class_name: "Person", foreign_key: :mentor_id
+
+  if ActiveRecord.gem_version >= Gem::Version.new('5.0')
+    belongs_to :mentor, class_name: "Person", foreign_key: :mentor_id, optional: true
+  else
+    belongs_to :mentor, class_name: "Person", foreign_key: :mentor_id
+  end
+
   has_paper_trail
 
   # Convert strings to TimeZone objects when assigned

spec/dummy_app/app/models/whatchamajigger.rb

@@ -1,4 +1,9 @@
 class Whatchamajigger < ActiveRecord::Base
   has_paper_trail
-  belongs_to :owner, polymorphic: true
+
+  if ActiveRecord.gem_version >= Gem::Version.new('5.0')
+    belongs_to :owner, polymorphic: true, optional: true
+  else
+    belongs_to :owner, polymorphic: true
+  end
 end

spec/dummy_app/app/models/wotsit.rb

@@ -1,6 +1,11 @@
 class Wotsit < ActiveRecord::Base
   has_paper_trail
-  belongs_to :widget
+
+  if ActiveRecord.gem_version >= Gem::Version.new('5.0')
+    belongs_to :widget, optional: true
+  else
+    belongs_to :widget
+  end
 
   def created_on
     created_at.to_date

spec/dummy_app/config/application.rb

@@ -34,6 +34,9 @@ class Application < Rails::Application
       if v >= Gem::Version.new("5.0.0.beta1")
         config.active_record.time_zone_aware_types = [:datetime]
       end
+      if v >= Gem::Version.new("5.1")
+        config.load_defaults 5.1
+      end
     end
   end
 end