Support for HTTP-date in HTTP Retry-After Header (RFC9110)

According to RFC9110 (https://httpwg.org/specs/rfc9110.html#field.retry-after) the Retry-After Header supports two types of value: HTTP-date and delay-seconds.
When the field contains a http-date, the check crashes and falls back to the default of 2 seconds sleep:

`.acme.sh/acme.sh: line 5875: [: Thu,20Aug202609: integer expression expected`

I'd be happy to provide a pull request if you'd like.

Thanks!

The issue points to line 5875 in acme.sh where the Retry-After header parsing fails for HTTP-date format. Start by examining the script around that line to see how the header value is currently processed. Compare with RFC9110 to understand the two formats (HTTP-date and delay-seconds). Write a fix that handles both formats correctly, then test with a server that returns an HTTP-date Retry-After header.

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