fix `test_counter_cache_column_update_on_destroy` for Rails 4.2

This is a workaround actually. Since
`ActiveRecord::CounterCache.each_counter_cached_associations` is
private. We shouldn’t use it. Proposals for the proper fix are welcome.

The reason the previous version stopped working is `affected_rows`
returns `0` at
[active_record/counter_cache.rb#L142](https://github.com/rails/rails/blo
b/ef99d4cd3ecc58a8c1484740b2fb5447dbda23ab/activerecord/lib/active_recor
d/counter_cache.rb#L142). If you follow the method call you’ll find
that it’s called at
[active_record/persistence.rb#L486](https://github.com/rails/rails/blob/
ef99d4cd3ecc58a8c1484740b2fb5447dbda23ab/activerecord/lib/active_record/
persistence.rb#L486). Probably we’d better override the return value in
`ActiveRecord::Relation.delete_all` at
[active_record/relation.rb#L481](https://github.com/rails/rails/blob/ef9
9d4cd3ecc58a8c1484740b2fb5447dbda23ab/activerecord/lib/active_record/rel
ation.rb#L481).