rotational-cipher: Prevent solutions that simply duplicate the alphabet

[katrinleinweber]

As discussed in https://github.com/exercism/problem-specifications/pull/1438 and other issues/PRs linked there, I would like to suggest that the R track adds test for rotation keys <0 and >26. What do you think?

Things I am unsure about:

• [ ] +1 the difficulty?
• [ ] bump exercise version?