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/WMTS.js

@@ -408,7 +408,7 @@ OpenLayers.Layer.WMTS = OpenLayers.Class(OpenLayers.Layer.Grid, {
                 path = path + this.matrixSet + "/" + this.matrix.identifier + 
                     "/" + info.row + "/" + info.col + "." + this.formatSuffix;
                 
-                if (this.url instanceof Array) {
+                if (OpenLayers.Util.isArray(this.url)) {
                     url = this.selectUrl(path, this.url);
                 } else {
                     url = this.url;