fix: [stix2 import] Fixed Custom Object type parsing
fix: [stix2 import] Fixed Custom Object type parsing

- Unlike usual STIX2 objects, Custom Objects do not
  have their own type. They are dict and have thus
  no callable attributes

SteveClement/MISP,devnull-/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP,Rafiot/MISP,FloatingGhost/MISP,MISP/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,MISP/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,devnull-/MISP,devnull-/MISP,MISP/MISP,Rafiot/MISP,SteveClement/MISP,SteveClement/MISP,Rafiot/MISP,devnull-/MISP,FIRSTdotorg/MISP,FIRSTdotorg/MISP,MISP/MISP,FloatingGhost/MISP,Rafiot/MISP