Does not work with flatten

```rust
#[test]
fn test_flattened_struct() {
#[derive(Deserialize, Debug)]
struct Package {
name: String,
#[serde(flatten)]
dependency: Dependency,
}

#[derive(Deserialize, Debug)]
struct Dependency {
version: String,
}

let j = r#"{
"name": "demo",
"version": 1
}"#;

// this doesn't work
test::(j, "version");
// this is the current result
// test::(j, ".");
}
```