Ruby 2.0 compatibility

Currently MimeMagic.by_magic isn't working in Ruby 2.0, I believe for problems which are described here: http://stackoverflow.com/questions/15843684/binary-string-literals-in-ruby-2-0

This PR should address the problem, while maintaining compatibility with ruby 1.9. I chose not to use the ruby 1.8 fix but I can submit a request with that instead if preferred.