Fix reply-by-mail for 8-bit transfer encodings

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.

lib/email/receiver.rb

@@ -217,6 +217,13 @@ module Email
       encodings = ["UTF-8", "ISO-8859-1"]
       encodings.unshift(mail_part.charset) if mail_part.charset.present?
 
+      # mail (>=2.5) decodes mails with 8bit transfer encoding to utf-8, so
+      # always try UTF-8 first
+      if mail_part.content_transfer_encoding == "8bit"
+        encodings.delete("UTF-8")
+        encodings.unshift("UTF-8")
+      end
+
       encodings.uniq.each do |encoding|
         fixed = try_to_encode(string, encoding)
         return fixed if fixed.present?