Every bill of lading in the hands of a consignee or endorsee for valuable consideration representing goods to have been shipped on board a vessel, shall be conclusive evidence of such shipment as against the master or other person signing the same, notwithstanding that such goods or some part thereof may not have been so shipped. unless such holder of the bill of lading shall have had actual notice at the time of receiving the same that the goods had not in fact been laden on board :

Provided that the master or other person so signing may exonerate himself, in respect of such misrepresentation, by showing that it was caused without any default on his part, and wholly by the fraud of the shipper or of the holder, or some person under whom the holder claims.