title: openlayers strategy: ol.loadingstrategy.bbox does'nt work with wfs features
body: <p>I tried to visualize parcels in wfs with openlayers 4 (max 1000) with this code.
  But the extent does not change at each movement or zoom of the view map, or the new features do not load in the layer?</p>
  
  <pre><code> var parcellaireSource =  new ol.source.Vector({ 
      format: new ol.format.GeoJSON(),
      url: function (extent) {
        return  'https://wxs.ign.fr/api key/geoportail/wfs?request=GetCapabilities&amp;SERVICE=WFS&amp;VERSION=2.0.0&amp;request=GetFeature&amp;typename=BDPARCELLAIRE-VECTEUR_WLD_BDD_WGS84G:parcelle&amp;outputFormat=application/json&amp;srsname=EPSG:2154&amp;bbox='+ extent.join(',') + ',EPSG:3857';
  
            },
  
      strategy: ol.loadingstrategy.bbox
      });
       var parcellairewfs =  new ol.layer.Vector({
      source: parcellaireSource
    });
       map.addLayer(parcellairewfs);
  </code></pre>