When gift may be suspended or revoked.—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be revoked in any of the cases (save want or failure of consideration) in which, if it were a contract, it might be rescinded. Save as aforesaid, a gift cannot be revoked. Nothing contained in this section shall be deemed to affect the rights of transferees for consideration without notice