String.mb_chars.upcase doesn't upcase non-ASCII chars on with Ruby 1.9.x

String#mb_chars does not currently return an instance of ActiveSupport::Multibyte::Chars under Ruby 1.9, leaving methods such as #upcase and #downcase up to Ruby 1.9's String class rather than a proxy.

At the time this code was developed, the assumption was that Ruby itself would provide this functionality in 1.9, so at the time the decision made perfect sense.

However, Matz has decided to leave UTF-8 case-folding out of Ruby 1.9, so there's currently no way to get UTF-8 aware upcasing/downcasing with Rails and Ruby 1.9:

Ruby 1.8.7:

"café".mb_chars.upcase # "CAFÉ"

Ruby 1.9.x

"café".mb_chars.upcase # "CAFé"

The patch I will attach makes AS use a proxy class on Ruby 1.9 to make the behavior consistent across Ruby versions. I also refactored all of AS's Unicode support into a new Unicode module to group related functionality and make it easier to reuse outside of ActiveSupport.