Found the original commit https://github.com/rails/rails/commit/ae8070e21be405331a1ad97f59c4e43d45f10eb3 :)

@josevalim can you c/d the "Hash keys must be strings" comments in this file? The [tests](https://github.com/rails/rails/commit/4df96338ede62da8ae8c188de5454b4b6b186ff8#diff-c751398f031cfa34a4c63083df4b7d4fR7) were using symbols since the beginning, so it's probably okay to relax this requirement?