This might be an ActiveSupport / Rails specific issue, but I'm not sure.
In JRuby 1.7.2 (1.9 mode), Rails 3.2.11:
In Ruby 1.9.3, Rails 3.2.11:
See also flori/json#152
flori/json#155 is a new request that attempts to fix this problem.
This problem is now fixed in the current master, so you can check it out. It will be released soon.
Sounds like 1.7.7 is out so this should be resolved now. Closing.