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/Tile/Image.js

@@ -373,7 +373,7 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile, {
             }
 
             // needed for changing to a different server for onload error
-            if (this.layer.url instanceof Array) {
+            if (OpenLayers.Util.isArray(this.layer.url)) {
                 this.imgDiv.urls = this.layer.url.slice();
             }