wallet endpoints validating tx buffers should expect hex

[bucko13]

Sending raw tx's, for example when hitting the sign endpoint, give an error that they expect a buffer. Since JSON doesn't really recognize Buffers and just stringifies the object, what is actually expected is the hex version of the transaction. We should either update bval.buf to check for stringified version of a Buffer or add support for bval.hex and/or improve error messaging to be more clear about the expectation.

See here: https://github.com/bcoin-org/bcoin/blob/master/lib/wallet/http.js#L508