Fix RateLimitRejectedException

src/Polly/RateLimit/RateLimitRejectedException.cs

@@ -60,6 +60,7 @@ public RateLimitRejectedException(TimeSpan retryAfter, String message, Exception
         private void SetRetryAfter(TimeSpan retryAfter)
         {
             if (retryAfter < TimeSpan.Zero) throw new ArgumentOutOfRangeException(nameof(retryAfter), $"The {nameof(retryAfter)} parameter must be a TimeSpan greater than or equal to TimeSpan.Zero.");
+            RetryAfter = retryAfter;
         }
 
         private static string DefaultMessage(TimeSpan retryAfter) => $"The operation has been rate-limited and should be retried after ${retryAfter}";