Add back set_inverse_instance on .add_to_target

We must have it in there too, so when an existent record is being concat to another,
we will have the inverse relation.

activerecord/lib/active_record/associations/collection_association.rb

@@ -350,6 +350,7 @@ module ActiveRecord
         end
 
         callback(:after_add, record)
+        set_inverse_instance(record)
 
         record
       end