Optional validation does not work anymore

The following schema

``` javascript
schema({
  a: [Function, String],
  b: String,
  c: Array.of(3, String),
  d: Array.of(3, String),
  e: [null, schema.self]
})
```

will never pass on object without the e field.

The issue it is on the latest, not the 0.6.2
