Curl#curl_headers: Work with 56 exit_status

[samford]

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I previously added the 8 curl exit code (weird server reply) to the list of non-success exit codes that #curl_headers will handle. We're now seeing failures with a 56 exit code (failure in receiving network data), where the server returns a 4xx response for a HEAD request but the same request using GET works as expected (e.g., casks like beeper, get-api, odrive, ui, etc.).

This adds 56 to the list of exit codes in #curl_headers, so a response with a 4xx HTTP status will be automatically retried using GET.

[p-linnane]

Thanks!

[MikeMcQuaid]

Thanks @samford!