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/HTTPRequest.js

@@ -209,7 +209,7 @@ OpenLayers.Layer.HTTPRequest = OpenLayers.Class(OpenLayers.Layer, {
         // in which case we will deterministically select one of them in 
         // order to evenly distribute requests to different urls.
         //
-        if (url instanceof Array) {
+        if (OpenLayers.Util.isArray(url)) {
             url = this.selectUrl(paramsString, url);
         }