title: Dynamically changed url in my Vector layer protocol not taken into account after refresh
body: <p>I have a map which I try to change dynamically its url when the user clicks a radio button.</p>
  
  <p>Here is my code :</p>
  
  <pre><code>var refreshLayer = new OpenLayers.Strategy.Refresh({ force: true, active: true });
  var fromProjection = new OpenLayers.Projection("EPSG:4326");
  var toProjection = new OpenLayers.Projection("EPSG:3857");
  var vectorProtocol = new OpenLayers.Protocol.HTTP({
          url: '/getmaindata',
          format: new OpenLayers.Format.GeoJSON({
              'internalProjection': fromProjection,
              'externalProjection': toProjection
          }),
          params: {
              "naf": "04"
          }
      });
  var map = new OpenLayers.Map("map_element");
  var vectorStrategies = [new OpenLayers.Strategy.Fixed(), refreshLayer];
  var osmLayer = new OpenLayers.Layer.OSM();
  var vectorLayer = new OpenLayers.Layer.Vector('vector_layer', {
     protocol: vectorProtocol,
     strategies: vectorStrategies
  });
  
  map.addLayers([osmLayer, vectorLayer]);
  
  (...)
  
  // $radio is my radiobutton
  $radio.onclick = function() {
        vectorProtocol.url = "/getouterdata";
        refreshLayer.refresh({force: true});
  });
  </code></pre>
  
  <p>When I debug with Firebug, I notice that the url is correctly changed in my layer's protocol object. However, what is called is the previous url (/getmaindata) instead of the new one (/getouterdata).
  So I was asking myself if there was something I did wrong. When is the new url called ? Is it by calling the refresh() method of my Strategy.Refresh ? Or somewhere else ? Maybe the old url is cached ?</p>