Add test for ES256

[maxbeizer]

👋 from a new user.

I was attempting to use joken to sign EC private keys and I can't seem to get it to work. I didn't see a test for ES256 so I figured I'd add it. I've included a burned private key in the tests (I have deleted it from the source).

Anyway, I'm back to debugging but thanks for this library. I assume my problem is PEBKAC but if you know of any reason why the following appears to always return 401 in the request for me, I'm all 👂 🙇

Joken.Signer.create(
  "ES256",
  %{"pem" => "--------BEGIN EC PRIVATE KEY-----\nM..."
)

cc @victorolinasc