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

• https://github.com/graylog-labs/gelf-rb/pull/64 (original PR)
• https://github.com/graylog-labs/gelf-rb/pull/73 (another approach but revoked)
• https://github.com/fluent/fluentd/issues/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