message: Fix Retry-After header check
  This failed to be triggered because the response has all its headers as lower case
diff: @@ -31,7 +31,7 @@ bool GetResponseHeaderBasedDelay(RawResponse const& response, Delay& retryAfter)
  return true;
  }
  - if ((header = responseHeaders.find("Retry-After")) != responseHeadersEnd)
  + if ((header = responseHeaders.find("retry-after")) != responseHeadersEnd)
  {
  // This header is in seconds.
  retryAfter = std::chrono::seconds(std::stoi(header->second));
  