Support more than one level of inheritance for 'subModelTypes' (Polymorphic collections/HasMany)

[jrreed]

I've run into an issue using subModelTypes for Polymorphic collections/HasMany.

Here is a jsFiddle demonstrating my issue and the fix I came up with:
http://jsfiddle.net/jimbo/HVDpQ/

My issue is that if one of the classes listed in subModelTypes is extended, and defines its own a set of subModelTypes as well, it doesn't get properly inflated from the JSON coming from the server.

So if you're using the following classes:

    window.Zoo = Backbone.RelationalModel.extend({
        relations: [{
            type: Backbone.HasMany,
            key: 'animals',
            relatedModel: 'Animal',
            collectionType: 'AnimalCollection'
        }]
    });

    window.Animal = Backbone.RelationalModel.extend({
        subModelTypes: {
            'mammal': 'Mammal'
        }
    });

    window.AnimalCollection = Backbone.Collection.extend({
        model: Animal
    });

    window.Mammal = Animal.extend({
        subModelTypes: {
            'dog': 'Dog'
        }
    });

    window.Dog = Mammal.extend();

If you then try to create a new Zoo like so:

    var zoo = new Zoo({
        animals: [{type: 'dog'}]
    });

I would expect zoo.get("animals").first() to be an instance of Dog but its actually an instance of Animal.

I did some poking around a bit and I found that the Backbone.RelationalModel.build function is only searching the subModelTypes defined in the top-level class. I'm thinking that it should probably check the subModelType attributes of its entire class hierarchy too:

Backbone.RelationalModel = Backbone.RelationalModel.extend({}, {
    build: function (attributes, options) {
        // 'build' is a possible entrypoint; it's possible no model hierarchy has been determined yet.
        this.initializeModelHierarchy();

        // Determine what type of (sub)model should be built if applicable.
        // Lookup the proper subModelType in 'this._subModels'.
        var model = this._findSubModelType(this, attributes) || this;
        return new model(attributes, options);
    },
    _findSubModelType: function (type, attributes) {
        if (type._subModels && type.prototype.subModelTypeAttribute in attributes) {
            var subModelTypeAttribute = attributes[type.prototype.subModelTypeAttribute];
            var subModelType = type._subModels[subModelTypeAttribute];
            if (subModelType) {
                return subModelType;
            } else {
                for (subModelTypeAttribute in type._subModels) {
                    return this._findSubModelType(type._subModels[subModelTypeAttribute], attributes);
                }
            }
        }
        return null;
    }
});

[jrreed]

I'd be happy to put together the tests and pull-request for adding this functionality. I just need to know if this would be a welcome addition if it is the right way to go about it.