### 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)


parent.setName("zhangsan");
        list.add(parent);
        result.setResult(list);
        Object generializeObject = PojoUtils.generalize(result);
        Object realizeObject = PojoUtils.realize(generializeObject, ListResult.class);
        assertTrue(realizeObject instanceof ListResult);
        ListResult listResult = (ListResult) realizeObject;
        List l = listResult.getResult();
        assertEquals(1, l.size());
        assertTrue(l.get(0) instanceof Parent);
        Parent realizeParent = (Parent) l.get(0);
        Assertions.assertEquals(parent.getName(), realizeParent.getName());
        Assertions.assertEquals(parent.getAge(), realizeParent.getAge());
    }
    @Test
    public void testListPojoListPojo() throws Exception {
        InnerPojo<Parent> parentList = new InnerPojo<Parent>();
        Parent parent = new Parent();
        parent.setName("zhangsan");
        parent.setAge(Integer.MAX_VALUE);
        parentList.setList(Arrays.asList(parent));
        ListResult<InnerPojo<Parent>> list = new ListResult<InnerPojo<Parent>>();
        list.setResult(Arrays.asList(parentList));
        Object generializeObject = PojoUtils.generalize(list);
        Object realizeObject = PojoUtils.realize(generializeObject, ListResult.class);
        assertTrue(realizeObject instanceof ListResult);
        ListResult realizeList = (ListResult) realizeObject;
        List realizeInnerList = realizeList.getResult();
        Assertions.assertEquals(1, realizeInnerList.size());
        assertTrue(realizeInnerList.get(0) instanceof InnerPojo);