Wrong model serialization

```javascript
var Foo = Backbone.AssociatedModel.extend({});
var Bar = Backbone.AssociatedModel.extend({});

var Baz = Backbone.AssociatedModel.extend({
            relations: [
                {
                    type: Backbone.One,
                    key: 'fooRel',
                    relatedModel: Foo,
                    serialize: ['name']
                },
                {
                    type: Backbone.One,
                    key: 'barRel',
                    relatedModel: Bar,
                }
            ],
});

var foo = new Foo({name: 'John', age: 30});
var bar = new Bar({test: 1});

var baz = new Baz({'fooRel': foo, 'barRel': bar});

var json = baz.toJSON();
// => {fooRel: {name: 'John'}, barRel: {}} //bug.
```
Should be as follows: ``` {fooRel: {name: 'Jonh'}, barRel: {test: 1}} ```
This occurs due to overwriting `options`.