Sync #initialize override to latest rails implementation

It's too bad this needs to get synced every time there's a change in ActiveRecord's implementation, but this syncs up `#initialize` after this changed was committed: https://github.com/rails/rails/commit/558f3204463eea80dc32b6748fe8b4174ec7ff14

Would be great if we could work with the rails project for a better implementation, but this is a quick win for now.