Translate event for an authorizer input.

def _prepare_authorizer_event(self, arn, lambda_event, lambda_context):
        # type: (str, EventType, LambdaContext) -> EventType
        """Translate event for an authorizer input."""
        authorizer_event = lambda_event.copy()
        authorizer_event['type'] = 'TOKEN'
        try:
            authorizer_event['authorizationToken'] = authorizer_event.get(
                'headers', {})['authorization']
        except KeyError:
            raise NotAuthorizedError(
                {'x-amzn-RequestId': lambda_context.aws_request_id,
                 'x-amzn-ErrorType': 'UnauthorizedException'},
                b'{"message":"Unauthorized"}')
        authorizer_event['methodArn'] = arn
        return authorizer_event