Freezed union toJson doesn't include unionKey with a single subclass

[BuyMyBeard]

Describe the bug
Whenever a sealed class only has a single subclass, it won't add the union key to the toJson.

To Reproduce

import 'package:freezed_annotation/freezed_annotation.dart';

part 'main.freezed.dart';
part 'main.g.dart';

@freezed
sealed class SingleSubClass with _$SingleSubClass {
  factory SingleSubClass.x() = SingleSubClassX;
	
  factory SingleSubClass.fromJson(Map<String, dynamic> json) =>
			_$SingleSubClassFromJson(json);
}

@freezed
sealed class TwoSubClasses with _$TwoSubClasses {
  factory TwoSubClasses.x() = TwoSubClassesX;
  factory TwoSubClasses.y() = TwoSubClassesY;
	
  factory TwoSubClasses.fromJson(Map<String, dynamic> json) =>
			_$TwoSubClassesFromJson(json);
}

void main() {
  print('single: ${SingleSubClass.x().toJson()}'); // single: {}
  print('two: ${TwoSubClasses.x().toJson()}'); // two: {runtimeType: x}
}

Expected behavior
If the class is declared as a Sealed class, it automatically adds the union key with toJson, even if it possesses a single subclass.

[rrousselGit]

That's not really a bug. Freezed doesn't need the key to decode the object, so it doesn't add it.

You could add it youself fairly easily by overriding toJson