Signature in PaymentRequest from Coinbase fails to verify

[matiu]

details in the PR.

[braydonf]

I've taken a look at this, and noticed bad signatures with other wallets as well, however updating to the most recent versions it was fixed. I think we should verify the test fixtures against other implementations, such as https://github.com/bitcoinj/bitcoinj to verify that the signature is indeed valid. And possibly add some additional fixtures.