fix #251

fix the issue 
#251
add params node into request when args is null or empty .

Author: anysoft

src/main/java/com/googlecode/jsonrpc4j/JsonRpcClient.java

@@ -343,6 +343,7 @@ private ObjectNode internalCreateRequest(String methodName, Object arguments, St
 		addParameters(arguments, request);
 		addAdditionalHeaders(request);
 		notifyBeforeRequestListener(request);
+		addNoneArguments(request);
 		return request;
 	}
 
@@ -423,6 +424,13 @@ private boolean isCollectionArguments(Object arguments) {
 		return arguments != null && Collection.class.isInstance(arguments);
 	}
 
+	private void addNoneArguments(ObjectNode request){
+		if (!request.has(PARAMS)){
+			//for none params add an empty array
+			request.set(PARAMS,mapper.valueToTree(new String[0]));
+		}
+	}
+	
 	private void addCollectionArguments(Object arguments, ObjectNode request) {
 		Collection<?> args = Collection.class.cast(arguments);
 		if (!args.isEmpty()) {