Retry-After header in 429 Too Many Request responses should be time until rate limit reset

[andy-paine]

Issue

The Retry-After header should be time until reset rather than the current implementation of Unix time at which the rate limit reset

Context

The Mozilla docs cover this fairly well https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After. We currently just set the value to be the same as the X-RateLimit-Reset header value

Steps to Reproduce

1. Query a CC API instance with a rate limit enabled
2. Make enough requests to exhaust limit
3. Look at headers on 429 response

Expected result

1. X-RateLimit-Reset header contains timestamp at which limit resets
2. Retry-After header contains number of seconds until limit resets

Current result

1. X-RateLimit-Reset header contains timestamp at which limit resets
2. Retry-After header contains timestamp at which limit resets

Possible Fix

Calculate the diff between the X-RateLimit-Reset and Time.now for the Retry-After header