Change advertised property test in gs_slurp to account for gsconfig variations

There are a differences in the 'advertised' property default value settings in Featuretype and Coverage in different gsconfig versions out there.  Trunk defaults to 'true' whereas 0.6.7 uses True.  Anyway, either should add these extra checks in or release a new version of gsconfig with latest changes in trunk.

This applies when GeoServer REST API is lacking the <advertised> element.