FIX: Use saved_change_to_value? in site_setting_saved event

Since Rails 5.2, the behavior of `attribute_changed?` inside `after_save` callbacks has changed, so we need to use `saved_change_to_attribute` instead. The site setting local_process_provider in test mode was covering up the issue.

lib/site_settings/local_process_provider.rb

@@ -8,7 +8,7 @@ class SiteSettings::LocalProcessProvider
     attr_accessor :name, :data_type, :value
 
     def value_changed?
-      true
+      false
     end
 
     def saved_change_to_value?