Fixes: spring-projects/spring-integration#2261
* Fix `ScatterGatherHandler.stop()` to call `GatherEndpoint.stop()` instead of `start()`
Adding unit test as per artembilan's suggestion
Replacing asterisk import with specific namespaces
Fixing formatting/codestyle errors shown by travis
Creating separate bean for new unit test
* Some simple polishing
* Merge the assertions for the fix into an existing test
**Cherry-pick to 4.3.x**
f202b25775