JIRA: https://jira.spring.io/browse/INT-4193
When using the internal `SequenceAwareMessageGroup` within an correlating
message handler, the messages were copied to a new collection before
checking for duplicate sequences in `canAdd()`.
This was unnecessary since we never add anything to this group, if `canAdd()`
returns true, the message is added to the store; this group is discarded.
Instead, use the message collection from the original group; although it is
not modifiable, this is not an issue because we don't need to modify it.
Conflicts:
spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java
spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroupFactory.java
src/reference/asciidoc/aggregator.adoc
* Polishing `aggregator.adoc` to reflect reality
318bb4c4b7