transaction object should have commit and rollback functions

[ricardoapuGL]

Hello!

While using the mock we encountered that Sequelize.transaction() it returns an empty object and so the test fails on t.commit()

https://github.com/Foyer-Inc/sequelize-mock/blob/97c54ea984787946888101d19ad536f3c31dccb6/src/sequelize.js#L391