throttling: fix RateLimit-Reset header
The RateLimit-Reset header was using the wait()
method from Django
REST Framework. This method computes the suggested time to wait before
the next request to avoid hitting the rate limit.
Instead, we use the duration
(width of the rate limiting bucket) and add
the timestamp of the first known request to it.
Thanks to @marmoute for the report.
Edited by Nicolas Dandrimont