allow postmaster to set owner and/or responsible

Signed-off-by: Michiel Beijen <michiel.beijen@otrs.com>
add unittests for new X-OTRS-(Owner|Responsible) handling

Signed-off-by: Michiel Beijen <michiel.beijen@otrs.com>
add sample files for unittests

Signed-off-by: Michiel Beijen <michiel.beijen@otrs.com>