Conversion of fields to text while creating mail merge documents
returns a cached value. This needs to be set before also for db name fields.

Change-Id: I5796adc82e11005ea045fb220b81201db5ff0b28
Reviewed-on: https://gerrit.libreoffice.org/19806Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Oliver Specht <oliver.specht@cib.de>