Section 7-507. Where a person negotiates or transfers a document of title for value otherwise than as a mere intermediary under the next following section, then unless otherwise agreed he warrants to his immediate purchaser only in addition to any warranty made in selling the goods
(a) that the document is genuine; and
(b) that he has no knowledge of any fact which would impair its validity or worth; and
(c) that his negotiation or transfer is rightful and fully effective with respect to the title to the document and the goods it represents.