More robust array type check. p=mwootendev, r=me (closes #2959)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@12095 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/TileCache.js

@@ -138,7 +138,7 @@ OpenLayers.Layer.TileCache = OpenLayers.Class(OpenLayers.Layer.Grid, {
         ];
         var path = components.join('/'); 
         var url = this.url;
-        if (url instanceof Array) {
+        if (OpenLayers.Util.isArray(url)) {
             url = this.selectUrl(path, url);
         }
         url = (url.charAt(url.length - 1) == '/') ? url : url + '/';