Zero count with nonzero density

[e-n-f]

@miles-dev alerts me to a feature in

{ "zoom": 16, "x": 3987, "y": 28823 }

that has

{ "density": 3, "count": 0 }

even though a count of 0 should always also correspond to a density of 0.

[e-n-f]

This is a multipolygon feature so it is based on density instead of absolute count. Looks like I missed the ceil in this case of extrapolating a count from a density.

[e-n-f]

Fixed in #35