export_xml generates incorrect data time format

[apoigne]

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export_xml generates a dateTime value such as '2021-09-22 00:00:00+00:00'. Should be '2021-09-22T00:00:00+00:00' according to w3 standards (noticed trying to import generated xml to SIOME)

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opcua-asyncio Version (e.g. master branch, 0.9):

[schroeder-]

xmlexport has a problem with exporting values, until we support the whole xml dataencoding, every change is just a small patch on a big problem.