simple-cipher: Disallow mixed case key

[tekerson]

This test has already been added to the TypeScript version of this exercise. The JavaScript example implementation already passes, but I still think the test is valuable as I've seen the same error in JavaScript student submissions.

A solution that checks if the key is valid (i.e. only contains alpha ascii lower-case letters) by testing

if (key === key.toUpperCase()) { throw new Error('Bad key') }`

passes the current "throws an error with an all caps key" test but allows mixed-case keys, which should actually be invalid.

[matthewmorgan]

Thanks @tekerson !

I was wondering about this, and I see that the canonical test file for this exercise does not contain a test against mixed-case keys, but the README specifies that the key should be all lowercase:

Let's make your substitution cipher a little more fault tolerant by providing a source of randomness and ensuring that the key contains only lowercase letters.

We appreciate you adding this here, can I suggest that you add it to problem-specifications also? It would help all the tracks.

[tekerson]

Good point @matthewmorgan!
I've created a PR to add the test to the canonical test file for the problem.

[matthewmorgan]

Awesome! Thanks for doing that. I'm not a maintainer on that track, but they tend to be pretty speedy about giving feedback.