Getting error when finding tile covers for extremely small polygons

Nobody has claimed this yet.

Here is the code snippet: https://jsfiddle.net/dfzaq1m2/ to request tile cover

It fails for both of these features:
- http://www.openstreetmap.org/way/490470366
- http://www.openstreetmap.org/way/490470364

Stack trace:
```
/Users/AmishaSingla/Documents/osm-compare/node_modules/tile-cover/index.js:229
if (ring && y === ring[0][1]) ring.pop();
^

TypeError: Cannot read property '1' of undefined
at lineCover (/Users/AmishaSingla/Documents/osm-compare/node_modules/tile-cover/index.js:229:30)
at polygonCover (/Users/AmishaSingla/Documents/osm-compare/node_modules/tile-cover/index.js:152:9)
at Object.getTiles [as tiles] (/Users/AmishaSingla/Documents/osm-compare/node_modules/tile-cover/index.js:77:9)
at feature_overlap (/Users/AmishaSingla/Documents/osm-compare/comparators/feature_overlap.js:111:19)
at /Users/AmishaSingla/Documents/osm-compare/tests/test_compare_function.js:23:3
at Array.forEach (native)
at Object. (/Users/AmishaSingla/Documents/osm-compare/tests/test_compare_function.js:22:19)
at Module._compile (module.js:409:26)
at Object.Module._extensions..js (module.js:416:10)
at Module.load (module.js:343:32)
```

cc @mourner

Reproduce the failure with the jsfiddle example and the two OpenStreetMap features, then trace tile-cover/index.js from getTiles through polygonCover and lineCover. Review comparators/feature_overlap.js and tests/test_compare_function.js for the calling and testing context. Done means extremely small polygons no longer throw and tile-cover returns a valid result.

Written by the indexing model from the issue text.