code: func (m *signingMethodNone) Sign(signingString string, key interface{}) (string, error) {
  	if _, ok := key.(unsafeNoneMagicConstant); ok {
  		return "", nil
  	}
  	return "", NoneSignatureTypeDisallowedError
  }
docs: // Only allow 'none' signing if UnsafeAllowNoneSignatureType is specified as the key