MAC keys insufficient requirement

[tanguilp]

According to RFC7518 - section 3.2:

A key of the same size as the hash output (for instance, 256 bits for "HS256") or larger MUST be used with this algorithm

seems to not be respected when MACing. For instance:

iex> JOSE.JWK.generate_key({:oct, 3}) |> JOSE.JWS.sign("some text", %{"alg" => "HS256"})
{%{alg: :jose_jws_alg_hmac},
 %{
   "payload" => "c29tZSB0ZXh0",
   "protected" => "eyJhbGciOiJIUzI1NiJ9",
   "signature" => "a_NlCa7P1CqQsSRPEydkTsMvMntCjl6pMvEPW7vr98s"
 }}

should fail.