INT-2634: Test for gateway's `Map` parameter

JIRA: https://jira.spring.io/browse/INT-2634

The Map entries with non-String keys are now skipped just with WARN message,
instead of `IllegalArgumentException` as it was before
INT-3200: SFTP: use `byte[]` for the PrivateKey

JIRA: https://jira.spring.io/browse/INT-3200

INT-3200: Remove reduntant test case

INT-3200: remove trailing space
Update CONTRIBUTING.md
Fix Occasional Reaper Expiry Test Failure

https://build.spring.io/browse/INT-MJATS41-32/

Also add a test for expiring a group within an aggregator.

Polishing