title: Format GPX.writeFeatures doesn't write feature nodes
body: <p>I'm attempting to generate a gpx from a LineString feature with ol/format/GPX.writeFeatures() but only get the gpx header:</p>
  
  <pre><code>&lt;gpx xmlns="http://www.topografix.com/GPX/1/1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd" version="1.1" creator="OpenLayers"/&gt;
  </code></pre>
  
  <p>with none of the feature's nodes.</p>
  
  <p>As a test, I've added:</p>
  
  <pre><code>let gpx = new GPX();
  console.log(features[0]);
  console.log(gpx.writeFeatures(features[0]));
  </code></pre>
  
  <p>to the <a href="https://openlayers.org/en/latest/examples/gpx.html" rel="nofollow noreferrer">GPX Data Example</a> index.js in the displayFeatureInfo function.</p>
  
  <p>Am I missing something?</p>