tracker3: Fix PUBLICATION_DATE writeback

be263953 introduced the use of the PUBLICATION_DATE key. The sparql
variable name for writeback however should be publicationDate to work.
The writeback resource builder should deal with the "publication date"
metadata key as well.

Related: !90
2 jobs for master in 3 minutes and 55 seconds
Status Job ID Name Coverage
  Test
passed build_stable #965545

00:03:09

passed build_tracker2 #965546

00:03:55