Clear all current instances before a reload.

If users added an attribute or otherwise changed a CurrentAttributes subclass
they'd see exceptions on the next page load.

Because `ActiveSupport::CurrentAttributes.current_instances` would keep
references to the old instances from the previous request.

We can fix this by clearing out the `current_attributes` before we unload
constants. Then any change to the model can be autoloaded again since its
slot isn't taken by an old instance.

We'll still have to call reset before we clear so external collaborators,
like Time.zone, won't linger with their current value throughout other code.

activesupport/lib/active_support/current_attributes.rb

@@ -126,6 +126,11 @@ def reset_all # :nodoc:
         current_instances.each_value(&:reset)
       end
 
+      def clear_all # :nodoc:
+        reset_all
+        current_instances.clear
+      end
+
       private
         def generated_attribute_methods
           @generated_attribute_methods ||= Module.new.tap { |mod| include mod }

activesupport/lib/active_support/railtie.rb

@@ -8,8 +8,9 @@ class Railtie < Rails::Railtie # :nodoc:
     config.eager_load_namespaces << ActiveSupport
 
     initializer "active_support.reset_all_current_attributes_instances" do |app|
-      app.executor.to_run      { ActiveSupport::CurrentAttributes.reset_all }
-      app.executor.to_complete { ActiveSupport::CurrentAttributes.reset_all }
+      app.reloader.before_class_unload { ActiveSupport::CurrentAttributes.clear_all }
+      app.executor.to_run              { ActiveSupport::CurrentAttributes.reset_all }
+      app.executor.to_complete         { ActiveSupport::CurrentAttributes.reset_all }
     end
 
     initializer "active_support.deprecation_behavior" do |app|