Coerce scale to a Number to make Mapnik happy
Coerce scale to a Number to make Mapnik happy

Without this, Mapnik emits `TypeError: optional arg 'scale' must be a number`.
CartoDB/tilelive-mapnik,wsw0108/tilesource-mapnik,dshorthouse/tilelive-mapnik,tomhughes/tilelive-mapnik,mapbox/tilelive-mapnik