Retry-After header, support non-standard formats

[mrienstra]

I encountered a website (powered by https://www.civicplus.com/civicengage/local-government-website-design ) which adds an s suffix, e.g. 30s instead of 30.

While looking into it, I found this thread, which discusses another non-standard format, 1m0s: urllib3/urllib3#1822

This PR adds support for both of these non-standard formats. AKA invalid formats, but given that it's better to correctly parse the value and respect it (vs. ignoring it), seems worth doing.

Decided to pull these changes out into a parseRetryAfter method for readability. Doing so made it easy to get rid of the nested ifs. Before:

    // The `retry-after` header can come in either <seconds> or
    // A specific date to go check.
    let retryAfter = Number(retryAfterRaw) * 1000 + Date.now();
    if (isNaN(retryAfter)) {
      retryAfter = Date.parse(retryAfterRaw);
      if (isNaN(retryAfter)) {
        // handle invalid response in formats `1m30s` or `30s`
        const matches = retryAfterRaw.match(/^(?:(\d+)m)?(\d+)s$/);
        if (matches) {
          retryAfter =
            (Number(matches[1] || 0) * 60 + Number(matches[2])) * 1000 +
            Date.now();
        } else {
          return false;
        }
      }
    }

After:

  // The `retry-after` header can come in either <seconds> or
  // A specific date to go check.
  private parseRetryAfter(retryAfterRaw: string): number {
    let retryAfter = Number(retryAfterRaw) * 1000 + Date.now();
    if (!isNaN(retryAfter)) return retryAfter;

    retryAfter = Date.parse(retryAfterRaw);
    if (!isNaN(retryAfter)) return retryAfter;

    // handle invalid response in formats `1m30s` or `30s`
    const matches = retryAfterRaw.match(/^(?:(\d+)m)?(\d+)s$/);
    if (!matches) return NaN;
    return (
      (Number(matches[1] || 0) * 60 + Number(matches[2])) * 1000 + Date.now()
    );
  }