Registration access token cannot be de-serialized when calling Client Configuration Endpoint

[ghost]

Because the initial registration access token saves the authorized scope in a SingletonSet when calling the Client Configuration Endpoint the access token cannot be deserialized because there's no Mixin registered.

A new mixin and custom deserializer should be registered in OAuth2AuthorizationServerJackson2Module

[jgrandja]

@ovidiupopa91 Instead of

A new mixin and custom deserializer

Changing Collections.singleton() to new HashSet() would be the easiest fix.

[ghost]

@jgrandja that's true. This is how it was initially, but in your polish commit you've changed it and I think it makes sense to have an immutable Set here. I can change it to Collections.unmodifiableSet as we already have a mixin registered for UnmodifiableSet