JIRA: https://jira.spring.io/browse/INT-3534
Previously the `MethodInvokingMessageListProcessor` copied `Collection<Message<?>>`
in its `process()` method for the `delegate` to a new `Collection` unconditionally.
To improve performance, the collection is no longer copied unconditionally
in the `ExpressionEvaluatingMessageGroupProcessor`.
Also add an `important` note to the `aggregator.adoc` about the restriction with `unmodifiableCollection`.
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