[@turf/line-slice-along] Fix offset distance equal to line length

[EricPKerr]

@turf/line-slice-along currently returns an invalid GeoJSON LineString if you pass in an offset that is equal to the line length.

The end of the module returns:

return lineString(coords[coords.length - 1]);

Which results in:

{
  type: 'Feature',
  properties: {},
  geometry: { type: 'LineString', coordinates: [ lon, lat ] }
}

This is a Point and not a valid LineString geometry (list of coordinates). The modification here returns the last point as a valid LineString geometry.

[mfedderly]

Thanks!