submodals with static throw error on create

``` javascript
var MyModel = new verymodel.Model({
id: {type: 'string', static: true},
owner: {model: {
owner_id: {type: 'string', static: true}
}}
});

var model = MyModel.create({
id: 'foobar',
owner: {
owner_id: 'fizzbuzz'
}
});

// Error: Cannot set static values
// at Object.owner_id (…/node_modules/verymodel/lib/model.js:347:27)
```

Identified in https://github.com/cloudflare/node-cloudflare/issues/2

Start with the reproduction in the issue and inspect lib/model.js around line 347, where the nested owner_id assignment raises the error. Verify the behavior with the shown MyModel.create call. Done means creating the model with static values in the nested submodel no longer throws the reported error.

Written by the indexing model from the issue text.