StubRunnerExecutor fails for Avro objects

[etrandafir93]

Describe the bug
StubRunnerExecutor::sendMessage converts the message to a JSON string before delegating to the MessageVerifierSender that should publish it:

void sendMessage(Contract groovyDsl) {
    // ...	

    Object payload = null;
    if (body != null && body.getClientValue() instanceof FromFileProperty) {
        FromFileProperty fromFile = (FromFileProperty) body.getClientValue();
        if (fromFile.isByte()) {
            payload = fromFile.asBytes();
        }
        else {
            payload = fromFile.asString();
        }
    }
    else {
        payload = JsonOutput
            .toJson(BodyExtractor.extractClientValueFromBody(body == null ? null : body.getClientValue()));
            // here! Even if "body" is an Avro object, it will be converted to json
    }

    this.messageVerifierSender.send(payload, headers == null ? null : headers.asStubSideMap(),
        outputMessage.getSentTo().getClientValue(), contract);
}

The MessageVerifierSender uses generic types, and it doesn't mention that the message comes as a JSON String.

When implementing the MessageVerifierSender::send, we'll most likely use an API for externalizing the message (eg: KafkaTemplate for Kafka). In this case, KafkaTemplate already encapsulates the logic for serializing the payload (as per the configured KafkaProperties).

In other words, the KafkaTemplate from MessageVerifierSender::send tries to serialize the payload as Avro, but it fails to do so (or ends up with a weird result) because the object passed to the MessageVerifierSender is a JSON string.

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Ugly workaround:
When implementing the MessageVerifierSender, I had to deserialize the JSON back to my Avro object, and (only then) let KafkaTemplate do the serialization.