cleaned up discourse_plugin

While working on a plugin, I found some low-hanging fruit.

* fixed whitespace
* added default `@events` hash and default `Set.new` key-values which removes need for explicit checks and simplifies the `discourse_event.rb` lib file.
* made the mixin search work in terms of the original comment in the file.
* added some tests to verify my changes.
* rearranged the DiscourseEvent spec into three context: nonexistent event, one event, multiple events.

This started as a typo correction so it's cool if I went too far. I fixed the typo, though. :chicken: