Return retry-after header when rate limit hits
Additionally, change sample Nginx file to set the error code to 429 (Too Many Requests) instead of its default 503 (Service Unavailable).
BREAKING CHANGE: The RateLimit method get_expiration_time
has been
renamed to compute_expiration_time
which is more clear and that name
has been used for the method that returns the expiration time for an
identifier.