This covers the core functionality of the handler.

Here are ways the tests could be further improved, which I deemed not worth the effort:

1. Testing if the handler is attached properly for Rails < 3 vs. >=3. I have tested it manually for the >=3 case and it works.

2. higher level functional test in the style of how DelayedJob is tested, where actual failing jobs are queued up:
  https://github.com/rollbar/rollbar-gem/blob/master/spec/rollbar/delayed_job_spec.rb

I didn't do this because it's more difficult to do with Sidekiq, for reasons explored here:
  sidekiq/sidekiq#2128

Style changes for tests.