Simple Cipher unit tests do not check for keys with non alphanumeric characters

[xarxziux]

I've just reviewed a submission for the Simple Cipher exercise where the student only checked for upper case letters and numbers in the key. This was enough to pass the unit tests, but does not conform to the requirements that state "Let's make your substitution cipher a little more fault tolerant by ... ensuring that the key contains only lowercase letters".

The unit tests for this exercise will need to be updated to test for this scenario.

[tejasbubane]

@xarxziux We maintain a canonical language-agnostic dataset of test cases for our exercises in this repo: https://github.com/exercism/problem-specifications. Can you create one issue there so it gets pushed to other languages as well?

[xarxziux]

I realised that after I created this issue and have already opened one:

exercism/problem-specifications#1298

[tejasbubane]

Closed via #448