Changes some of a contract job's custom fields (BF-7519).

Body: an array of { customFieldId, value }. Fields not listed are left as they are. value null or "" clears the field; a required field cannot be cleared. A list field takes an option id or name (any case); a name that matches two options needs the id. A user field takes the userId of an active user of your company. A free-form field takes up to 500 characters. Only changed values are checked and written; sending the stored values changes nothing. Responds with the job's custom fields as GET returns them.