Feature: sequelize.set method

Method to be analyzed on #2094
I don't have 100% sure but i think i have an error here.

Mocha is catching `Uncaught TypeError: Cannot call method 'bulkBuild' of null` , and if i change `null` to `undefined`, it gaves me `Uncaught TypeError: Cannot call method 'bulkBuild' of undefined`.

Any hint about this?