Serialize dates correctly for cache filling job

Date objects were not handled specially, causing json serialization to fail. They are now handled.

The context object has been changed to use dependency injection, making it much easier to get a context object appropriate for testing.

The part for fetching termin overvakning has also been wrapped in a try expect since the dfo-sap-client does not handle invalid dates well. For the time being, we simply ignore touching the cache for this part of the data. In the future we should get a newer version of the dfo-sap-client that does not care about this issue.