Ensure that duplicate proofs are pruned when issuing UCANs and that proofs are sorted

[Gozala]

We have recently encountered UCANs that have multiples of the same proof, which should not have happened, issuing a UCAN from this library should have treated proofs as a set.

[Gozala]

I think fix would have to go here

js-dag-ucan/src/lib.js

Line 156 in 21bae4e

prf: proofs,

and it would turn remove duplicates and sort cids.