API v3 contact update API should not check for required fields

API v3 is looking checking for required fields for contacts. For example when updating individuals it checks for 'first_name' and 'last_name' or 'email'. This of course doesn't make sense during an update since the update can act solely on other fields. Of course on the other hand if we don't do any verification someone could use the api to unset a required field, but that to check for that might be excessive.