This adds support for writing DateTime and DateTimeImmutable values with the XLSX writer. The dates are written using the date type (t="d") which must then specify an ISO date as the value (as opposed to a linear date in previous versions).
Adds support for DateTimeImmutable by using the shared DateTimeInterface which both this and DateTime implement.
This methods was untested, so this adds basic support for all data types currently tested by other test cases in this class.
Rhodri Pugh