"The duty of a shopkeeper in this class of case is well-established. It may be said to be a duty to use reasonable care to see that the shop floor, on which people are invited, is kept reasonably safe, and if an unusual danger is present of which the injured person is unaware, and the danger is one which would not be expected and ought not to be present, the onus of proof is on the defendants to explain how it was that the accident happened."