Only add hostname to In-Reply-To header when needed

When sending mail via Mantis email api, the In-Reply-To header was
treated differently from the others: the value provided was
concatenated with the hostname and enclosed in angular brackets,
which prevented setting a custom In-Reply-To.

We now only add the hostname when one is not already present.

Fixes 0029585, PR https://github.com/mantisbt/mantisbt/pull/1795

Co-authored-by: Daniele Verducci daniele.verducci@ag2.it