### Environment

* Dubbo version: 2.7.4.1
* Operating System version: macOS Catalina Version 10.15.3
* Java version: 1.8.0_231

### Steps to reproduce this issue

1. 通过telnet调用接口如下
方法(Method)
`public String convert7(UserInfoCondition userInfoCondition) {  }`
参数(Parameter)
`public class UserInfoCondition {
    private List<Map<Integer, UserInfo>> userList;
}`
`public class UserInfo {
    private String accountMobile;
    private String userPassword;
}`

invoke UsesrInfoFacade.convert7({"userList":[{"1":{"accountMobile":"15052234567"}}]})
or
invoke UsesrInfoFacade.convert7({"userList":[{1:{"accountMobile":"15052234567"}}]})

提示如下错误(has a problem)
![image](https://user-images.githubusercontent.com/17045745/77892951-ebd50d00-72a5-11ea-95b0-dd59a68a83cc.png)

到底如何书写这个入参呢?(How to write the parameters)


public DemoServiceImpl() {
        super();
    }
    public void sayHello(String name) {
        System.out.println("hello " + name);
    }
    public String echo(String text) {
        return text;
    }
    public Map echo(Map map) {
        return map;
    }
    public long timestamp() {
        return System.currentTimeMillis();
    }
    public String getThreadName() {
        return Thread.currentThread().getName();
    }
    public int getSize(String[] strs) {
        if (strs == null)