Wrong tile is returned for edge cases

Testcase:

```javascript
const cover = require('@mapbox/tile-cover');

const tilebelt = require('@mapbox/tilebelt');

const coordinates = [-11.250000000000002, 6.816667036613423];

console.log({ coordinates });

const tiles = cover.tiles({ type: 'Point', coordinates }, { min_zoom: 8, max_zoom: 8 });

console.log({ tiles });

const bbox = tilebelt.tileToBBOX(tiles[0]);

console.log({ bbox });

console.log({
inTile:
coordinates[0] >= bbox[0] &&
coordinates[0] <= bbox[2] &&
coordinates[1] >= bbox[1] &&
coordinates[1] <= bbox[3] &&
coordinates[0] !== null &&
coordinates[1] !== null,
});
```

Output:

```
{ coordinates: [ -11.250000000000002, 6.816667036613423 ] }
{ tiles: [ [ 120, 123, 8 ] ] }
{ bbox: [ -11.25, 5.615985819155343, -9.84375, 7.013667927566632 ] }
{ inTile: false }
```

Start with the cover.tiles entry point and reproduce the provided JavaScript example at zoom 8, then compare its tile with the bounding box from tilebelt.tileToBBOX. Done means the returned tile contains the supplied coordinates for this edge case and the behavior is covered by a regression test.

Written by the indexing model from the issue text.