@Override
    public void close(CompletionHandler handler) {
        closed = true;
        if (handler != null) {
            handler.completed();
        }
    }
@author <a href="mailto:einarmr@yahoo-inc.com">Einar M R Rosenvinge</a>
@since 5.1.9
container-core/src/test/java/com/yahoo/processing/rendering/TestContentChannel.java