tzinfo change breaks xml rendering

Reproduction instructions:

ruby script/generate scaffold user name:string
rake db:migrate
ruby script/runner "User.new(:name=>'George').save"
ruby script/server &
sleep 10
curl http://localhost:3000/users/1.xml

This was broken by commit 04eb2b72b400384fe49e31b95c11c68cab2aca8e. This also affects ActiveResource.