Fix RabbitAmqpAdmin.purgeQueue for proper return

The `Management.queuePurge()` now returns a `PurgeStatus` abstraction,
where the `messageCount` property is what we expect on the `AmqpAdmin` contract.

Related to: https://github.com/rabbitmq/rabbitmq-amqp-java-client/issues/151

spring-rabbitmq-client/src/main/java/org/springframework/amqp/rabbitmq/client/RabbitAmqpAdmin.java

@@ -402,9 +402,8 @@ public class RabbitAmqpAdmin
 	@ManagedOperation(description = "Purge a queue and return the number of messages purged")
 	public int purgeQueue(String queueName) {
 		try (Management management = getManagement()) {
-			management.queuePurge(queueName);
+			return (int) management.queuePurge(queueName).messageCount();
 		}
-		return 0;
 	}
 
 	@Override