The mail class seems to handle mails sent with Content-Transfer-Encoding: 8bit
somewhat weirdly: It decodes them (to utf-8), changes the raw source to base64,
and does not modify the Content-Type:charset= header.
This leads to Discourse trying the message encoding (in my example ISO-8859-1)
first, and if that does not contain any unparseable characters, it uses that.
Sadly, in ISO-8859-1, every byte sequence is valid.
Fix this by always trying to decode as UTF-8 first. The probability of someone
using another encoding that cleanly (but wrongly) decodes as UTF-8 should be
fairly low.
