Add constructor call to support delegate and method name

When configuring Spring Rabbit with pure Java config, it's easier with
a constructor call that lets you specify the delegate and the POJO method
name at once.

```java
	public MessageListenerAdapter(Object delegate, String defaultListenerMethod) {
		this(delegate);
		setDefaultListenerMethod(defaultListenerMethod);
	}
```

This constructor lets me setup java config like this:
```java
	@Bean
	MessageListenerAdapter listenerAdapter() {
		return new MessageListenerAdapter(new Receiver(), "receiveMessage");
	}
```

P.S. This patch includes automated unit tests to verify the new constructor
method signature.