add "default" argument to fallback OSFSXAttrMixin
add "default" argument to fallback OSFSXAttrMixin

Konubinix/pyfilesystem,flying-circus/pyfilesystem,piranna/pyfilesystem,rivenz/pyfilesystem,pombreda/pyfilesystem,pmitros/pyfs,PyFilesystem/pyfilesystem,zopyx/pyfilesystem,pmitros/pyfs