Store the feature's commonly used id

To not clobber the feature's attributes, this is a separate
member property.

src/ol/parser/geojson.js

@@ -129,6 +129,9 @@ ol.parser.GeoJSON.prototype.parseFeature_ = function(json, opt_options) {
       geometry = null,
       options = opt_options || {};
   var feature = new ol.Feature(json.properties);
+  if (goog.isDef(json.id)) {
+    feature.setFeatureId(json.id);
+  }
   if (geomJson) {
     var type = geomJson.type;
     var callback = options.callback;