Camel RabbitMQ invalid handling of message timestamp

At the moment the RabbitMQ component is does not map the timestamp of a message appropriately. The outbound mapping (producer) expects the timestamp of the camel message is of type String whereas the String is just the long value representing the timestamp. However the timestamp is already a java.util.Date when the producer just forwards a message from a rabbitmq consumer as the timestamp is already a java.util.date as define in AMQP.BasicProperties.

The provided pull request provides a compatible change. So it still keeps the old behaviour as fallback so that the long value is evaluated if the provided data is not a java.util.Date