Writer doesn't support writing DateTime objects

[guss77]

According to Box\Spout\Common\Entity\Cell, a DateTime object is a valid cell value, and it is correctly assigned the type TYPE_DATE. But it seems the writer implementations do not support it: when trying to write a sheet that contains a TYPE_DATE cell, the writer throws Box\Spout\Common\Exception\InvalidArgumentException: Trying to add a value with an unsupported type: object

(Happened in ODS writer but the code looks similar for XLS and looks like CSV just tries to stringify all the cells, which will not work because DateTime does not have a __toString() method).

[ChronicLogic]

I added a PR #781 for XLSX, I had addressed this separately, unaware that there was an issue with ODS or CSV. Whoever (if anyone) would integrate the PRs should look at all of the cases.

[adrilo]

Duplicates #662