* Removed `MultiJSON` dependency. As a result, `ActiveSupport::JSON.decode`
no longer accepts an options hash for `MultiJSON`. ([Pull Request](https://github.com/rails/rails/pull/10576) / [More Details](upgrading_ruby_on_rails.html#changes-in-json-handling))
* Removed support for the `encode_json` hook used for encoding custom objects into
JSON. This feature has been extracted into the [activesupport-json_encoder](https://github.com/rails/activesupport-json_encoder)