fix a bug in the json_data function of GradLeave class - a datetime object didn't convert to a string.
fix a bug in the json_data function of GradLeave class - a datetime object didn't convert to a string.

uw-it-aca/uw-restclients,UWIT-IAM/uw-restclients,uw-it-aca/uw-restclients,UWIT-IAM/uw-restclients,uw-it-cte/uw-restclients,UWIT-IAM/uw-restclients,uw-it-cte/uw-restclients,uw-it-cte/uw-restclients