Fix headers initialization on Thunderbird 91.7.0
Fix headers initialization on Thunderbird 91.7.0

The "Missing label for header" error
is caused by https://hg.mozilla.org/releases/comm-beta/rev/5b5ed57a340e

moisseev/rspamd-spamness,moisseev/spamness,moisseev/rspamd-spamness