Reading OOXML _x005F_ escaped content may not get unescaped, so
when writing back to OOXML do not attempt to escape it again, i.e.
write _x005F_xHHHH_ as is and not as _x005F_x005F_xHHHH_.
This is more a workaround, the proper fix would be to unescape
_x005F_ content upon read. But then the entire "invalid XML
character" escapement and handling control characters rat tail
would come into play.
Reviewed-by: Eike Rathke <email@example.com>