`readonlyDefault` for bytes field returns mutable list

Same bug for `map` fields: #705 

Repro:

```proto
syntax = "proto3";

message M1 {
  bytes b = 1;
}
```

```dart
import '../test.pb.dart';

void main() {
  M1 m = M1();
  m.b.add(0);
  m.b.add(1);
  print(m);
}
```

The last line prints empty bytes field.