Use original RFC822-formatted subject, if available (e.g. recv'd via SMTP)

This solves issue #817 for TLDR messages imported into DOVE-Net Tech Talk,
because those messages are imported via SMTP originally and thus have the
original (MIME/Q-encoded subject) in the message header (as RFC822SUBJECT).

For other messages with UTF-8 characters in their header fields (e.g. subject),
we'll need a different solution (Q-encode header fields that contain
non-ASCII chars on the fly), and then this solution could maybe be reverted.