Support 8bit transfer encoding

[sj26]

Currently MailCatcher does not advertise 8bit compatiblity, and only supports ASCII encoded mail bodies. But most SMTP servers these days support 8bit transfer encodings, and presume utf-8, so the SMTP implementation should probably catch up. There have been various reports of this over the years, but this is a central issue to track the feature request for 8bit transfer encoding support.