[server]: unsubscribe_call doesn't close subscriptions as intended

[niklasad1]

When an unsubscription call is received by the Server then RawServerSubscriptionId::from_wire_message tries to parse it from hardcoded JsonValue::Null where the subscription ID should be parsed instead. Thus, RawServerSubscriptionId::from_wire_message always fails to parse the subscription ID

This causes dropping a registered subscription not work with one exception when the entire client is dropped then it works via SubscriptionsClosed

[dvdplm]

Maybe unrelated but I think null is a valid JSON document.

[niklasad1]

Yeah, it is unrelated the problem is that JsonValue::Null is hardcoded instead the subscription ID should be parsed from the request/method call. I rephrased the description hopefully more understandable now

[niklasad1]

Fixed by #180