NaN printed in cosole as null

This code:

``` js
var schema = Joi.object().keys({
        date: Joi.date().allow(null)
});

Joi.assert({date: NaN}, schema);
```

prints in the console:

``` text
Error: {
  "date" [1]: null
}

[1] "date" must be a number of milliseconds or valid date string
```

and could be really confusing to debug!
