not to accept algorithm none by default

According to https://tools.ietf.org/html/draft-ietf-jose-json-web-algorithms#section-3.6, JWS implementations must not accept header with { "alg": "none" } by default.

>  Implementations that support Unsecured JWSs MUST NOT accept such
>  objects as valid unless the application specifies that it is
>  acceptable for a specific object to not be integrity protected.