to_json crash on MIRToStakeCredentials

[SebastienGllmt]

Any transaction that contains a MIR certificate crashes when calling to_json. This may be caused by our good old friend linked_hash_map

Crash log:

Err` value: JsError { msg: "to_json: key must be a string" }

Steps to reproduce

#[test]
    fn foo_bar() {
        let mut cred = MIRToStakeCredentials::new();
        cred.insert(
        &StakeCredential::from_keyhash(
            &Ed25519KeyHash::from_bytes(
                hex::decode("b861eeadde300385d88aaa98cad0f0ed1f95419bbb9971a0fb7c96fb").unwrap()
            ).unwrap()
        ),
        &Int::from_str("5").unwrap()
        );
        println!("{}", cred.to_json().unwrap());
    }

[SebastienGllmt]

I believe the root cause is a missing implementation of serde::Serialize for StakeCredential such as

impl serde::Serialize for TransactionMetadatum {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where S: serde::Serializer {
        let json_str = decode_metadatum_to_json_str(self, MetadataJsonSchema::DetailedSchema)
            .map_err(|e| serde::ser::Error::custom(&format!("{:?}", e)))?;
        serializer.serialize_str(&json_str)
    }
}

Note sure why the derived implementation doesn't give us what we want