Ensure event diff JSON is always of type str.

There was some confusion introduced when using the DeepDiff to_json serializer, where UPDATE events were always of type str, the interface being clients should deserialize, whereas CREATED events were relying on the default field value of dict, which when deserialized client side would fail,

TypeError: the JSON object must be str, bytes or bytearray, not dict

Make this consistent for both CREATED and UPDATED events, and cover the cases with better testing.