Empty object value in `create` does not override factory default

Given this factory:

``` javascript
const fact = unionized.factory({foo: {bar: 3}})
```

Creating an object like so:

``` javascript
fact.create({foo: {}})
```

The expected output object is

``` json
{
  "foo": {}
}
```

but the result is

``` json
{
  "foo": {
    "bar": 3
  }
}
```

This works as expected with `{foo: null}` and `{foo: undefined}`.
