Actually clarify previous test.

I've made wrong assertion (no comma and newline mixed), but it still passes.
Fix separator in spec.

TODO is missing newlines that http://osherove.com/tdd-kata-1/ has.
Make test with custom delimiters pass.
Make specs less random - test all possible custom delimiters.
Constants in rspec example groups are global for entire suite.

At least used to be so I avoid them.
Add failing spec for negative arguments.