Add test coverage for plain text fallback

This adds coverage for LOGSTASH-1595 as well (the binary blob test)
Make tests for LOGSTASH-1595 pass.
Add encoding and fallback coverage to json codec (LOGSTASH-1595)
Fix json encoding and callback tests (LOGSTASH-1595)
Merge pull request #920 from jordansissel/LOGSTASH-1595

Add encoding and fallback test coverage to json and json_lines codecs