Make Flat XML document name consistant with ODF
List discussion: http://lists.freedesktop.org/archives/libreoffice/2013-November/057337.html

Change-Id: I4ddaa7adcffefbedab895f5e94eb6bd92201386e
Reviewed-on: https://gerrit.libreoffice.org/7488
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>