This restored the functionality of Relation#join (e.g. .where(...).join(...)) to the expected Array behaviour for the 4.1 release series. If you have such code in your app, a temporary work around is to do add a to_a after the relation before 4.1.2 comes out.
The previous deprecated require path, active_support/core_ext/object/to_json has been removed from master. This is also a periodic reminder that you can use config.active_support.deprecation = :raise to aggressively find and clean up deprecated code (docs). Not something I'd recommend doing in production of course!
While improving the documentation, @schneems stumbled upon a bug in the code, and submitted a fix for it. If you are looking for ways to start contribute to Rails, this seems like an excellent path to follow!