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
```
+1 to being able to configure a charset encoder. For users who need to round-trip parsed emails, an exact encoding makes sense. For users who need a UTF-8 email at any cost, a "best effort" charset encoding makes sense.