Serialization Error when using UUID as pk on delete.

[kristofwybomos]

#100 Fixed some issues with using UUIDs as primary keys, but one issue remains. When trying to delete an object with UUID as pk on Master, there's a JSON serialization error. The issue occurs in MasterSignals classmethod post_delete. instance_data contains instance.pk which can be of type UUID. Later in TransportPayload, get_json_valid_value is used to get a string for instance_pk and correlation_id but not the contents of instance_data.