Normalize charset

On my system for some reason my locale wass set to pt_BR.utf8, and that makes Encoding.find() be called with "utf8", which is not how Ruby represents "UTF-8". The last commit in this branch will try harder to find a matching encoding amongts the ones supported natively by Ruby comparing the input charset name with modified versions of the Ruby encodings.

I have since figured out why my locale was set like that and was able to revert it to the expected charset name, but I think there could be a lot of other people with such locales out there.