I am using voluptuous 0.9.2 and I have a problem with Exclusive class. I need that if there is none of the keys, it should give an error. However, this is okay for voluptuous. Is this a bug of voluptuous? If not, how I can write a script for that?

In order to clarify my problem, assume we built a schema like that:

```
schema = Schema({Exclusive('a', 'z'): int, Exclusive('b', 'z'): int, 'c': int}, required=True)

```

I need one and only one of the keys in exclusion group to be given.
But when I test with `{'c': 5}`, it seems to be valid even though I did not give either `a` or `b`.
I do not know how to make it works especially for this situation.