changed behavior of add_header primitive to match P4 v1 spec: when the header becomes valid, fields are reset to 0
changed behavior of add_header primitive to match P4 v1 spec: when the header becomes valid, fields are reset to 0

hanw/behavioral-model,hanw/behavioral-model,ops-p4/ops-p4dp,hanw/behavioral-model,ops-p4/ops-p4dp,p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,ops-p4/ops-p4dp,p4lang/behavioral-model,ops-p4/ops-p4dp,hanw/behavioral-model,hanw/behavioral-model