Fix for multipe occurrences of attibute in <w:pgMar>

After round trip multiple occurrences of w:left,  w:right etc
were exported.
Also added UT for the test file.

Conflicts:
	sw/qa/extras/ooxmlexport/ooxmlexport.cxx

Change-Id: I6a503cf609f627b8f8eb9d33691e51582c15b459
Reviewed-on: https://gerrit.libreoffice.org/6608