Set the done flag once the task completes.

Author: David Ellingsworth

src/RestSharp/AsyncHelpers.cs

@@ -103,15 +103,12 @@ async void PostCallback(object? _) {
                 try {
                     // Do not call ConfigureAwait(false) here to ensure all continuations are
                     // queued on this context, not the thread pool.
-                    await _task();
+                    await _task().ContinueWith(_ => _done = true);
                 }
                 catch (Exception exception) {
                     _caughtException = ExceptionDispatchInfo.Capture(exception);
                     throw;
                 }
-                finally {
-                    Post(_ => _done = true, null);
-                }
             }
         }