- Treat negative Retry-After date-based times as 0.
- Treat Retry-After times greater than 6 hours as 6 hours.
Prior to this change Retry-After did not have a limited range and the
server could have set a time greater than 6 hours or a date in the past
that would result in a negative time, either of which may be unexpected
by the user.
The 6 hour limit is purposely not documented so that it can be changed
in the future if necessary.
Closes https://github.com/curl/curl/pull/15833
