EventSource: make eventSourceDeRegisteredForResource a default method

[lburgazzoli]

The EventSource is defined as follow:

public interface EventSource extends Closeable {
    default void start() {
    }

    default void close() {
    }

    void setEventHandler(EventHandler var1);

    void eventSourceDeRegisteredForResource(String var1);
}

I think that eventSourceDeRegisteredForResource should be make as a default method.

[metacosm]

Closed by #471