Include id in toGeoJSON result

Fixes #40

Author: jfirebaugh

lib/vectortilefeature.js

@@ -155,12 +155,18 @@ VectorTileFeature.prototype.toGeoJSON = function(x, y, z) {
         type = 'MultiLineString';
     }
 
-    return {
+    var result = {
         type: "Feature",
         geometry: {
             type: type,
             coordinates: coords
         },
         properties: this.properties
     };
+
+    if ('_id' in this) {
+        result.id = this._id;
+    }
+
+    return result;
 };

test/parse.test.js

@@ -72,6 +72,7 @@ test('parsing vector tiles', function(t) {
 
         var point = tile.layers.poi_label.feature(11).toGeoJSON(8801, 5371, 14);
         t.deepEqual(point.type, 'Feature');
+        t.deepEqual(point.id, 3000003150561);
         t.deepEqual(point.properties, {
             localrank: 1,
             maki: 'park',