Use Spree.user_class instead of Spree::LegacyUser

Spree::LegacyUser is only used in the dummy app for testing so replaced it with Spree.user_class

Author: JustShah

api/app/controllers/spree/api/base_controller.rb

@@ -68,7 +68,7 @@ def permitted_line_item_attributes
       end
 
       def permitted_user_attributes
-        can?(:admin, Spree::LegacyUser) ? super + admin_metadata_attributes : super
+        can?(:admin, Spree.user_class) ? super + admin_metadata_attributes : super
       end
 
       def load_user