Treat xml:space specially in OOXMLFastContextHandler::startFastElement,
to allow this attribute to be handled for any element.

Change-Id: I81bd1e0642940ffdfc03d6c65d0ce9f421206c5e
Reviewed-on: https://gerrit.libreoffice.org/70723Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>