Change order of params: passed params should override existing params.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@559 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/Grid.js

@@ -235,7 +235,7 @@ OpenLayers.Layer.Grid.prototype = Object.extend( new OpenLayers.Layer(), {
         var requestString = "";        
         this.params.SRS = this.map.projection;
         // concat tile params with layer params and convert to string
-        var allParams = Object.extend(params, this.params);
+        var allParams = Object.extend(this.params, params);
         var paramsString = OpenLayers.Util.getParameterString(allParams);
 
         var server = this.url;