Update tests (and fix plural rule parsing)

Firstly there was bug made when `abs` was used as it wouldn't work for passed strings (it would use 0 then)
Next, tests weren't updated to match changes in numbering system and rule parsing. And also there's no place for `deep_merge` in `rules.rb` as it's not used there but in `export.rb`
Now I fixed those and also noticed that plural rules still weren't properly implemented as `f` and `t` should be integers and thus `t` for "0.020" would be `2`. I implemented it a bit shorter/cleaner and also added more tests specially for actual rule checking.
Hopefully now it will work correctly.

When new gem version could be released? All tests does pass so I think it should be fine.

BTW @camertron you might want to take a look