'Class specific implementation of from_jwt which should take jwt components and return
and instance of this Class with jwt information loaded.
:return: Jwt object containing the headers, payload and key'

@classmethod
def _from_jwt(cls, headers, payload, key=None):

    jwt = Jwt(secret_key=key, issuer=payload.get('iss', None), subject=payload.get('sub', None), algorithm=headers.get('alg', None), valid_until=payload.get('exp', None), nbf=payload.get('nbf', None))
    jwt.__decoded_payload = payload
    jwt.__decoded_headers = headers
    return jwt
