Rails 3.2.x is now compatible with Ruby 2.0.0

This PR is pretty much a bunch of backport commits since we already fixed a lot of stuff in `master`. This will making sure that you'll be able to use your Rails 3.2.x project with Ruby 2.0.0 that just came out today.

All tests are green: https://travis-ci.org/rails/rails/builds/5027376

And yes, happy 20th birthday Ruby!

![credit: http://clintandelyse.blogspot.com/2009/09/happy-birthday-ruby.html](https://lh3.ggpht.com/_kYHP9FOYrMk/Sqki2JlGBaI/AAAAAAAAAiw/nPPkrFqylAQ/s400/IMG_1772.JPG)