Fix ODF roundtrip of "center accross selection". [#726010]

2014-03-12  Andreas J. Guelzow <aguelzow@pyrshep.ca>

	* openoffice-write.c (odf_write_style_cell_properties):
	GNM_HALIGN_CENTER_ACROSS_SELECTION should be source="fix"
	(odf_write_style_paragraph_properties): use "center" for
	GNM_HALIGN_CENTER_ACROSS_SELECTION