rails/rails — pull request #16300 by vast (2014-07-25 21:07:25+00:00)
Title: Mention changes to `nil` handling in serialized attributes in "Upgrading from Rails 3.2 to Rails 4.0"

The change was introduced in https://github.com/rails/rails/commit/39cb1bedb82814d7bf9de6834f88c31e4df27278.