Add `NaN` support back in to collectd codec

* Add spec tests for NaN support
* Catch warning on NaN with nan_handling set to warn
* Make lambdas ivars and initialize so they can reference other ivars
* Fixed reference to point to ivar
* Streamline ivar declarations in one method
* Improve tests to cover more cases
* Make lambdas not be ivars
* Change idiomatic calling of init_lambdas to reflect lack of return value

closes 1363
Merge git://github.com/logstash/logstash