fixed toString override in IE8 (fixed issue #1850)

genjs.ml

@@ -1155,6 +1155,7 @@ let generate com =
 		print ctx "function $extend(from, fields) {
 	function inherit() {}; inherit.prototype = from; var proto = new inherit();
 	for (var name in fields) proto[name] = fields[name];
+	if( fields.toString !== Object.prototype.toString ) proto.toString = fields.toString;
 	return proto;
 }
 ";