Add json_dump method and use that for converting to json #3

[scrwr]

As there are 2 PRs being stalled and the issue is unsolved since a long time. This is a third try to patch it.

References are

• Add json_dump method and use that for converting to json #64 (original PR)
• Replace explicit Yajl support with MultiJson #73 (another approach but revoked)
• Multi line issue within labels fluent/fluentd#2404 (effected parties)

[CLAassistant]

All committers have signed the CLA.

[scrwr]

The build issue doesn't seem related to my change.
Gem::GemNotFoundException: can't find gem bundler (>= 0.a) with executable bundle
only for Ruby: jruby-19mode