Mail merge wasn't copying the document properties into the target
document. It also has to copy most document properties into the
working copy, as ConvertFieldsToText is called just there.

Change-Id: I7e07fec712c225f667d960a94f4eb3fd0b83f041
Reviewed-on: https://gerrit.libreoffice.org/10987Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>