fix: [stix2 import] Fixed timestamp parsing
fix: [stix2 import] Fixed timestamp parsing

- Fixed timestamp parsing of custom objects

SteveClement/MISP,MISP/MISP,MISP/MISP,SteveClement/MISP,MISP/MISP,MISP/MISP,SteveClement/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP