protected IEnumerable<Bundle> CreateExternalBundlesUrlReferences(Bundle[] bundlesArray)
 {
     var existingUrls = bundlesArray.OfType<IExternalBundle>().Select(b => b.Url);
     var generator = new ExternalBundleGenerator(existingUrls, settings);
     foreach (var bundle in bundlesArray)
     {
         bundle.Accept(generator);
     }
     return generator.ExternalBundles;
 }