INT-3499 AMQP Confirms/Returns Polishing

JIRA: https://jira.spring.io/browse/INT-3499

- Suppress (log) 'null' payload error when no correlation data
- Change tests to use a dedicated template for confirms/returns

Conflicts:
	spring-integration-amqp/src/test/java/org/springframework/integration/amqp/config/AmqpOutboundChannelAdapterParserTests.java
	src/reference/docbook/amqp.xml

Resolved.