Datetime in the headings row causes TypeError

[Bjwebb]

This leads to some quite inscrutable error messages in CoVE.

e.g.

Error message: TypeError('datetime.datetime(2023, 12, 15, 0, 0) is not JSON serializable')

It's probably best to just cast as a string.

Internal reference: https://opendataservices.plan.io/issues/47141