Symbol isn't being attached

https://runkit.com/embed/jngz13x7dgdl

```js
var fluent = require("fluent-schema")
var schema = fluent
    .object()
    .prop('foo', fluent.string().enum(['foo']))
    .prop('bar', fluent.array()).valueOf()
console.log(schema[Symbol.for('fluent-schema-object')] === true)
```

Results in `false` being logged.