buttonClickSelector must have data-url attribute

If no data url button which is like a form child button, it'll failure to fire correct handler.

So, I fixed the behavior.

Following example is not "buttonClickSelector".
```
<form action="/purchase" method="post">
  <input type="hidden" value="item_id_or_something">
  <button data-confirm="are you ready?">
</form>
```

Following example is "buttonClickSelector".
```
// this button is not form child
<button data-confirm="are you ready?" data-url="/purchase", data-method="post">
```