/**
 * Invokes the given method with the given arguments and invokes the
 * {@code JsonRpcCallback} with the result cast to the given
 * {@code returnType}, or null if void. The {@code extraHeaders} are added
 * to the request.
 *
 * @param methodName   the name of the method to invoke
 * @param argument     the arguments to the method
 * @param extraHeaders extra headers to add to the request
 * @param returnType   the return type
 * @param callback     the {@code JsonRpcCallback}
 */
@SuppressWarnings("unchecked")
private <T> Future<T> doInvoke(String methodName, Object argument, Class<T> returnType, Map<String, String> extraHeaders, JsonRpcCallback<T> callback) {
	
	String path = serviceUrl.getPath() + (serviceUrl.getQuery() != null ? "?" + serviceUrl.getQuery() : "");
	int port = serviceUrl.getPort() != -1 ? serviceUrl.getPort() : serviceUrl.getDefaultPort();
	HttpRequest request = new BasicHttpEntityEnclosingRequest("POST", path);
	
	addHeaders(request, headers);
	addHeaders(request, extraHeaders);
	
	try {
		writeRequest(methodName, argument, request);
	} catch (IOException e) {
		callback.onError(e);
	}
	
	HttpHost target = new HttpHost(serviceUrl.getHost(), port, serviceUrl.getProtocol());
	BasicAsyncRequestProducer asyncRequestProducer = new BasicAsyncRequestProducer(target, request);
	BasicAsyncResponseConsumer asyncResponseConsumer = new BasicAsyncResponseConsumer();
	
	RequestAsyncFuture<T> futureCallback = new RequestAsyncFuture<>(returnType, callback);
	
	BasicHttpContext httpContext = new BasicHttpContext();
	requester.execute(asyncRequestProducer, asyncResponseConsumer, pool, httpContext, futureCallback);
	
	return (callback instanceof JsonRpcFuture ? (Future<T>) callback : null);
}