Autoload thread safety

Autoload on Rubinius is not thread safe, test gist: https://gist.github.com/5e08c309ad8b69b2a31b

This passes on JRuby 1.7, MRI 2.0 and fails on MRI 1.8, MRI 1.9.3, Rubinius.

I can turn this into rubyspec (targeting 2.0?) if you guys want.