[failing] test edgeline

![geometry-collection](https://cloud.githubusercontent.com/assets/83384/4872812/6ac5c474-61f4-11e4-9fde-4ce977274f56.png)

This test fails because the intersection between tile and geometry ends up as a GeometryCollection upstream in `turf-intersect` and this check prevents any legit tiles from continuing on with a GeometryCollection:

- https://github.com/mapbox/tile-cover/blob/master/index.js#L95

A refactor of this to handle GeometryCollection is pretty ugly, and a better fix for this may be the work in  scanline which appears to handle this case more robustly.