Add default string conversion for dates in JSON responses

[DanielRyanSmith]

This change fixes a deployment bug that affected API responses.

TypeError: Object of type date is not JSON serializable

OpenAPI's to_dict() method will convert any value that is labeled with format: date to a date or datetime object, which is not JSON serializable by default. This can be solved more broadly by adding the default=str argument when converting to a JSON string, which denotes that any field that is not JSON serializable should be first type cast to a string.

Additionally, some of the items in the response are labeled as format: date, which drops any time granularity from the date. This is usually not needed, but I've converted timestamp-like fields to format: date-time. Other fields, like shipped_date, have been kept as date formats.