Verify default_header_lookup works for all cases
It is possible for a packet to be GROUP_STANDALONE
and not have a secondary header, e.g., /mnt/sdata/sipsingest/archive/2015/253/snpp/atms/sess/P1570513AAAAAAAAAAAAAS15253144442801.PDS
.
Make sure this case, or any other, does not get a secondary header impl.