[oldlogstashjson] add tests before making changes
[oldlogstashjson] encode to json instead of ruby hash (LOGSTASH-1626)
[oldlogstashjson] fix test assertions

Still do not fully grok the insist blocks or rspec testing,
but at least we're really making assertions now.
[oldlogstashjson] less convoluted not nil assertions
Merge pull request #788 from bfritz/LOGSTASH-1626-fix-oldlogstashjson-codec

LOGSTASH-1626 - encode to JSON instead of ruby hash