Test EmptyLinesAroundClassBody on singleton class
Spec for Token#to_s

Token#to_s was introduced in https://github.com/bbatsov/rubocop/pull/8
and seems untested forever. I assume it was introduced for debugging
purposes.
Test RedundantSelf with class methods, too
Test LiteralInCondition on post-loop while/until
Spec for passing an ambiguous formatter name
Don't check if the configuration option is nil

The default value for AllowedVariables (per default.yml) is `[]`.
Merge pull request #1530 from bquorning/increase-test-coverage

Increase test coverage