code: function( xhr, url ) {
  		try {
  			var xhrRes = xhr.getResponseHeader("Last-Modified");
  
  			// Firefox always returns 200. check Last-Modified date
  			return xhr.status == 304 || xhrRes == jQuery.lastModified[url];
  		} catch(e){}
  		return false;
  	}
docs: Determines if an XMLHttpRequest returns NotModified