Why does MessageSerializer have to encode a dict only?

There are cases in a Kafka key, for example, that is a simple `{"type": "string"}` rather than a record. It would still be helpful to use the MessageSerializer. However, it doesn't permit non-dict's because of this check

```
        if not isinstance(record, dict):
            raise SerializerError("record must be a dictionary")
```

Any reason for this? Can it be removed?

Thanks.
