Fix ToReadOnlyReactiveProperty() behavior

[AnnulusGames]

The current code path of ToReadOnlyReactiveProperty() is as follows.

1. When calling ToReadOnlyReactiveProperty() on ReadOnlyReactiveProperty<T>, the instance method is called. This returns the instance itself.

2. When calling ToReadOnlyReactiveProperty() on Observable<T>, the extension method is called. Here, if the source is ReadOnlyReactiveProperty<T>, the instance is returned as is.

The problem here is that if the source is ReadOnlyReactiveProperty<T>, even if initialValue and equalityComparer are specified as arguments, they are ignored. This is unintuitive and not desirable behavior.

Therefore, this PR changes the code path as follows.

1. When calling ToReadOnlyReactiveProperty() on ReadOnlyReactiveProperty<T>, the instance method is called.

2. When calling ToReadOnlyReactiveProperty() on Observable<T>, the extension method is called. Here, if source is ReadOnlyReactiveProperty<T> and it is the overload without arguments, the instance is returned as is.

This ensures that an instance is always created when an argument is specified, so it will work correctly even if source is ReadOnlyReactiveProperty<T>.

[AnnulusGames]

However, as pointed out in #263, it might be better to always create a new instance.