add failing test for all-day endpoint

@icambron, exactly as you suspected, including midnight of the next day in all day ranges is breaking some of our code. Since you mentioned that the change makes Twix more consistent and its code simpler, this PR only adds a failing test.

But basically, it seems like the following shouldn't be true:

```js
moment.twix("2013-04-12", "2013-04-12", true).contains("2013-04-13")
```

Let me know if I can help with this at all!