fix+optimize attribute handling in tile_datasource_pbf (fixes node-mapnik test failure)
fix+optimize attribute handling in tile_datasource_pbf (fixes node-mapnik test failure)

tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile,landsurveyorsunited/mapnik-vector-tile,landsurveyorsunited/mapnik-vector-tile,tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile,tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile,mapbox/mapnik-vector-tile,tomhughes/mapnik-vector-tile,landsurveyorsunited/mapnik-vector-tile