Allow TopoJSON features with id equal to 0

src/ol/format/topojsonformat.js

@@ -260,7 +260,7 @@ ol.format.TopoJSON.readFeatureFromGeometry_ = function(object, arcs,
   var feature = new ol.Feature();
   feature.setGeometry(/** @type {ol.geom.Geometry} */ (
       ol.format.Feature.transformWithOptions(geometry, false, opt_options)));
-  if (object.id) {
+  if (object.id !== undefined) {
     feature.setId(object.id);
   }
   if (object.properties) {

test/spec/ol/format/topojson.test.js

@@ -22,6 +22,16 @@ var aruba = {
   ]
 };
 
+var zeroId = {
+  type: 'Topology',
+  objects: {
+    foobar: {
+      type: 'Point',
+      id: 0,
+      coordinates: [0, 42]
+    }
+  }
+};
 
 describe('ol.format.TopoJSON', function() {
 
@@ -60,6 +70,15 @@ describe('ol.format.TopoJSON', function() {
       ]);
     });
 
+    it('can read a feature with id equal to 0', function() {
+      var features = format.readFeaturesFromObject(zeroId);
+      expect(features).to.have.length(1);
+
+      var feature = features[0];
+      expect(feature).to.be.a(ol.Feature);
+      expect(feature.getId()).to.be(0);
+    });
+
   });
 
   describe('#readFeatures()', function() {