The format used the ISO_OFFSET_DATE_TIME is the wrong one and leads to errors
formatting the date. Instead we should have used the ISO_LOCAL_DATE_TIME instead.
This commit fixes that and also includes a bug fix for the XLSX variant as that didn't
use the overridden method. The correct method for this has been overridden now also
formatting the dates/times for that using the ISO_LOCAL_DATE_TIME format.
Tests for different types have been added as well, as to make sure this
keeps working for newer versions.
Closes: #121
cc72e8d616