Deep subModelTypes with custom subModelTypeAttribute

[matghaleb]

subModel's instantiation is failing when using 2 or more levels of subModelTypes and custom subModelTypeAttribute.

You can see this issue from this jsfiddle and compare the results with my PR from here.

Here is the new test I joined to the PR. Please tell me if you need more information

test( "Object building based on type in a custom field, when using explicit collections" , function() {
            var scope = {};
            Backbone.Relational.store.addModelScope( scope );

            scope.Mammal = Animal.extend({
                subModelTypes: {
                    'primate': 'Primate',
                    'carnivore': 'Carnivore'
                }
            });
            scope.Primate = scope.Mammal.extend();
            scope.Carnivore = scope.Mammal.extend({
                subModelTypeAttribute: 'family',
                subModelTypes: {
                    'feline': 'Feline',
                    'canid': 'Canid'
                }
            });

            scope.Feline = scope.Carnivore.extend();
            scope.Canid = scope.Carnivore.extend();

            var MammalCollection = AnimalCollection.extend({
                model: scope.Mammal
            });

            var mammals = new MammalCollection( [
                { id: 5, species: 'chimp', type: 'primate' },
                { id: 6, species: 'panther', type: 'carnivore' },
                { id: 9, species: 'coyote', type: 'carnivore', family: 'canid' },
                { id: 10, species: 'leopard', type: 'carnivore', family: 'feline'}
            ]);

            ok( mammals.at( 0 ) instanceof scope.Primate );
            ok( mammals.at( 1 ) instanceof scope.Carnivore );
            ok( mammals.at( 2 ) instanceof scope.Canid );
            ok( mammals.at( 3 ) instanceof scope.Feline );
        });