Make url available as a public attribute

This is particularly useful if the real GET url is not unknown because additional parameters are appended via `send({param: 1})`. With this change you can assign the right url to `pushState`. 

```javascript
var request = new Request.HTML({
onSuccess: function(){
	window.history.pushState({ turbolinks: true, url: request.url }, '', request.url);
}
}).send({param: 1});
```