`relation.serialize` support an array of keys and particular key

The current version of the BB when specifying relation.serialize = `string`, we obtain the following:
```javascript
var Foo = Backbone.AssociatedModel.extend({});

var Bar = Backbone.AssociatedModel.extend({
            relations: [
                {
                    type: Backbone.One,
                    key: 'rel',
                    relatedModel: Foo,
                    serialize: 'id'
                }
            ],
});

var foo = new Foo({id: 111});
var bar = new Bar({rel: foo});

bar.toJSON() // { rel: { id: 111 } }
```
But I would like to get {rel: 111}.

This PR solves this problem.