Move canonicalization into RT::ObjectCustomFieldValue->Create
Previously, value canonicalization happened for dates and datetimes in RT::CustomField->AddValueForObject, and for IP addresses and ranges in RT::ObjectCustomFieldValue->Create. Standardize on the latter, as it is lower down and thus harder to skip accidentally. This requires creating the CustomField object as the current user, not as the system user, as Date and DateTime canonicalization cares about the time zone of the current user. This, in turn, requires switching to _Value('Type') instead of Type, as the constructed $cf object is not guaranteed to have the right context objects for it to be readable by the user.