fix a behavior of the cancel with two buttons.

I call this style alert view.
It calls a block even if the "Nevermind" was selected.

```ruby
UIAlertView.alert("This is happening, OK?", buttons: ["Nevermind", "OK"],
  message: "Don't worry, it'll be fine.") {
  self.happened!
}
```

I guess this fix is a right way although I have not tested other styles well.