Fix Ed25519 and X25519 JWK

[kenta-wakasa]

I would like to merge this change because Ed25519 and X25519 JWK is wrong.

Please refer to rfc8037#section-2.

Correct Example Ed25519 Private JWK

{
  "kty" : "OKP",
  "crv" : "Ed25519",
  "d" : "nWGxne_9WmC6hEr0kuwsxERJxWl7MmkZcDusAxyuf2A", // private key 
  "x" : "11qYAYKxCrfVS_7TyWQHOg7hcvPapiMlrwIaaPcHURo" // public key
}

Correct Example X25519 Public JWK

{
  "kty" : "OKP",
  "crv" : "X25519",
  "kid" : "Bob", // optional
  "x" : "3p7bfXt9wbTTW2HC7OQ1Nz-DQ8hbeGdNrfx-FG-IK08" // public key
}