more encodings and their equivalents

our users use a bunch of things ruby does not support, after a bit of googling most of them are very similar to ISO_8859-X

not sure if there is a better place for there tests ...

@bf4 @jeremy 
@grosser No need to repeat the same discussion again: https://github.com/mikel/mail/pull/772#discussion_r15857134

Picking "close" encodings doesn't fly.