v2.2.6.RELEASE

Change log:

39f5215 Upgrade dependencies; prepare for release
a587b13 Fix docs for batch error handler
40b21c1 GH-1066: Protect against null Headers
04f4db3 GH-1059: Add SpEL support for KafkaListener.id()
95f7c67 Fix close producers after a rebalance
0a7f4a9 Fix generated doc location
5601017 Doc update for scala 2.12