SerialSubscription is missing isUnsubscribed

[headinthebox]

public class SerialSubscription implements Subscription {
@OverRide
public void unsubscribe() {...}}
public void setSubscription(final Subscription subscription) {...}
public Subscription getSubscription() {...}
}

http://msdn.microsoft.com/en-us/library/system.reactive.disposables.serialdisposable_properties(v=vs.103).aspx

[benjchristensen]

Fixed in #593