INT-3423 Fix Expression-Based Splitter

JIRA: https://jira.spring.io/browse/INT-3423

Consider a message containing "one one" and a splitter
with expression "payload.split(' ')", only one message
is emitted.

The default Collection created by SpEL is a HashSet.

Change the ExpressionEvaluatingSplitter to expect a
List instead of a Collection.