The donor and donee may agree that on the happening of any specified event which does not depend on the will of the donor, a gift shall be suspended or revoked, but a gift which the parties agree shall be revocable wholly or in part, at the mere will of the donor, is void wholly or in part, as the case may be.A gift may also be revoked in any of the cases in which, if it were a contract, it might be rescinded.