bnc#885548: Reset the default user to the document owner at the end.

Otherwise the user name is set to the name of the user in the last revision
record, which will be used when recording new revisions.

Change-Id: I9b5e39c3b3a85614af67f86db9ed2bb3509a9e2b
(cherry picked from commit 227d3e72b0b29b73440157b64d6fb56fa151e05e)
Reviewed-on: https://gerrit.libreoffice.org/10610
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>