### 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)


Assertions.assertNull(realizedParent.email);
    }
    @Test
    public void testMapField() throws Exception {
        TestData data = new TestData();
        Child child = newChild("first", 1);
        data.addChild(child);
        child = newChild("second", 2);
        data.addChild(child);
        child = newChild("third", 3);
        data.addChild(child);
        data.setList(Arrays.asList(newChild("forth", 4)));
        Object obj = PojoUtils.generalize(data);
        Assertions.assertEquals(3, data.getChildren().size());
        assertSame(data.getChildren().get("first").getClass(), Child.class);
        Assertions.assertEquals(1, data.getList().size());
        assertSame(data.getList().get(0).getClass(), Child.class);
        TestData realizadData = (TestData) PojoUtils.realize(obj, TestData.class);
        Assertions.assertEquals(data.getChildren().size(), realizadData.getChildren().size());
        Assertions.assertEquals(data.getChildren().keySet(), realizadData.getChildren().keySet());
        for (Map.Entry<String, Child> entry : data.getChildren().entrySet()) {
            Child c = realizadData.getChildren().get(entry.getKey());
            Assertions.assertNotNull(c);
            Assertions.assertEquals(entry.getValue().getName(), c.getName());
            Assertions.assertEquals(entry.getValue().getAge(), c.getAge());
        }
        Assertions.assertEquals(1, realizadData.getList().size());
        Assertions.assertEquals(data.getList().get(0).getName(), realizadData.getList().get(0).getName());
        Assertions.assertEquals(data.getList().get(0).getAge(), realizadData.getList().get(0).getAge());
    }