Fix json encoding and callback tests (LOGSTASH-1595)
merge https://github.com/elasticsearch/logstash/pull/920/commits
Make tests for LOGSTASH-1595 pass.
Add test coverage for plain text fallback

This adds coverage for LOGSTASH-1595 as well (the binary blob test)
use json codec to decode gelf messages, follow-up on LOGSTASH-1595
create 1.2.2-syn4 tag