also fix a problem in handling this parameter when rsyslog processed
internal messages itself (it did not work). As the parameter was
introduced today, we do not flag this follow-up commit as "bugfix".
The issue was noticed when we added the additional tests.
see also https://github.com/rsyslog/rsyslog/issues/3650
see also https://github.com/rsyslog/rsyslog/issues/3639