The writeFeatures method should always return a string

src/ol/format/gpxformat.js

@@ -876,9 +876,15 @@ ol.format.GPX.prototype.writeFeatures;
 
 
 /**
- * @inheritDoc
+ * Encode an array of features in the GPX format as an XML node.
+ *
+ * @param {Array.<ol.Feature>} features Features.
+ * @param {olx.format.WriteOptions=} opt_options Options.
+ * @return {Node} Node.
+ * @api
  */
 ol.format.GPX.prototype.writeFeaturesNode = function(features, opt_options) {
+  opt_options = this.adaptOptions(opt_options);
   //FIXME Serialize metadata
   var gpx = ol.xml.createElementNS('http://www.topografix.com/GPX/1/1', 'gpx');