Cleanup specs for Kernel.#warn
Add specs on Kernel#warn for 2.0

https://github.com/rubinius/rubinius/pull/2500#pullrequestreviewcomment-5416715
Fix Kernel.#warn for 2.0
Merge pull request #2500 from kachick/2.0-kernel-warn

Kernel.#warn should accept multiple messages as Kernel.#puts from 2.0