fix(core/config): inherit is ignored when explicit default is present

[PhotonQuantum]

Fix a bug when both inherit and explicit default are set. Inherited fields was overwritten by explicit defaults.

Example

Source

    #[derive(Deserialize, Config)]
    struct Nested {
        #[config(default = "false")]
        b: bool,
        c: usize,
    }

    #[derive(Deserialize, Config)]
    struct Config {
        #[config(inherit, default = r#"{ "c": 42 }"#)]
        a: Nested,
    }

Current Generated Code (Wrong)

Notice how the second insertion of key "a" overwrites the first one.

impl crate::utils::ConfigDefault for ConfigWithInheritAndExplicitDefaultsTwo {
    fn config_defaults() -> crate::utils::serde_json::Value {
        crate::utils::serde_json::Value::Object({
            {
                let mut dict = crate::utils::serde_json::Map::new();
                dict.insert("a".to_string(), <Nested as crate::utils::ConfigDefault>::config_defaults());
                dict.insert("a".to_string(), crate::utils::serde_json::from_str::<crate::utils::serde_json::Value>("{ \"c\": 42 }").expect("Given string literal is not a valid json value."));
                dict
            }
        })
    }
}

Stacked PR

This PR is stacked on top of PR #145. It shouldn't be merged until #145 is merged.

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