Make sure shortcut methods don't colorize when Paint.mode is zero

I looked into #12 further and realised that it was only the shortcut methods that had this problem.

With this pull request I've added test coverage for the shortcut method when `Paint.mode` is zero, and I've also tidied up the shortcut specs (the `include` specs were commented out, and the `method_missing` specs didn't have the correct `should` syntax so weren't actually asserting anything).