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.

Change-Id: I3d31dc84a362753c23a8c89f7a5d7bfd06e4367b
Reviewed-on: https://gerrit.libreoffice.org/73187
Tested-by: Jenkins
Reviewed-by: Eike Rathke <erack@redhat.com>