support windows-1258

fixes https://github.com/mikel/mail/issues/544

Fix:
```Ruby
module Windows1258
  def pick_encoding(enc)
    case enc
    when /windows-?1258/i
      'Windows-1252'
    else
      super
    end
  end
end
class << Mail::Ruby19
  prepend Windows1258
end
```
Perhaps we could support an easier hook for end-users to add their own mappings in the meantime?