Small refactor to json_warning.rb

Hi, thanks for this gem. Started using it today and it's looking good.

I *thought* there was a bug in the falsy ||= assignment in this module, but after writing a spec to exercise it, I realised it worked okay.

I thought I'd push my tests and a small refactor anyway, in case you think the coverage is useful and you think the slightly-refactored module is an improvement in terms of readability.

Cheers,
Stu