Loading an invalid ODF without version attribute in manifest:manifest should add it

[svanteschubert]

The ODS test https://github.com/tdf/odftoolkit/blob/master/odfdom/src/test/java/org/odftoolkit/odfdom/doc/table/TableCellTest.java loads the (invalid) test document TestSpreadsheetStyleTable.ods, changes it, and saves it back, but there is still a version attribute missing in the manifest:

tested with https://odfvalidator.org/

Such a missing version attribute should be automatically added when it does not exist. This might happen on the DOM level, and I will take this issue to generate this feature.