RabbitMQMessage coder can't be registered when using Google Cloud Dataflow

When using Google cloud Dataflow, coders registered through
```

pipeline.getCoderRegistry().setCoderForClass(...)
```

Don't seem to be really registered. As a consequence, RabbitMqMessage objects are not correctly serialized.

I would love to provide more details, unfortunatly I was not able to produce a valid debugging session to dive into details.

Imported from Jira [BEAM-7433](https://issues.apache.org/jira/browse/BEAM-7433). Original Jira may contain additional context.
Reported by: riduidel.