Add plain codec to mirror the fact that it is bundled in logstash

[wiibaa]

Found while cleaning up dependencies in a plugin that is not using codec => logstash-plugins/logstash-output-statsd#25

@ph With seconds thoughts, IMHO it should maybe better go into logstash-plugin-core-api gemspec, no?

[ph]

This one is a real pain, I've encountered this issue in the past with other plugin.
I wish our plugin system didn't have any dependencies on plugins to correctly work. but the way codec currently work it wont be possible.

I believe this dependency is needed by the base classes used by the plugins where a default codec is defined. See https://github.com/elastic/logstash/blob/master/logstash-core/lib/logstash/inputs/base.rb#L40 (should be the same as the output type)

So if we track where is needed it's actually the logstash core that requires that dependency. I think it would be better that the core actually define that dependency?

[wiibaa]

@ph as long as the transitive dependencies make it through soon and allow correct execution of bundle exec rspec, I can continue to clean up plugins useless dependencies, it's a win-win ;)

[kares]

PR should no longer be relevant esp. with the move to test plugins using a LS installation.