AsyncLoading progress value doesn't propagate when setting state from AsyncLoading to AsyncLoading

[r52]

Describe the bug
AsyncLoading.progress doesn't propagate when setting state from AsyncLoading to AsyncLoading.

To Reproduce

@riverpod
Future<Model> example(ExampleRef ref) async {
  ref.state = AsyncLoading(progress: 0);   // <-- doesn't work
  ref.state = AsyncLoading(progress: 0.5); // <-- doesn't work

  await something();

  ref.state = AsyncLoading(progress: 1);   // <-- doesn't work
}

final test = const AsyncValue.loading(progress: 0.1).copyWithPrevious(const AsyncValue.loading(), isRefresh: false);
assert(test.progress == 0.1); // <-- fails, is null

Expected behavior
The new progress values are set.