core: Make the property name match the metadata key name
core: Make the property name match the metadata key name

GRL_METADATA_KEY_PUBLICATION_DATE should really correspond to
"publication-date" not "publishing-date".

https://bugzilla.gnome.org/show_bug.cgi?id=741605

jasuarez/grilo,grilofw/grilo,MathieuDuponchelle/grilo,grilofw/grilo,GNOME/grilo,MathieuDuponchelle/grilo,MathieuDuponchelle/grilo,jasuarez/grilo,jasuarez/grilo,grilofw/grilo,GNOME/grilo