Caesar test case suggestion

[lnevesabrantes]

I wrote a version of the Caesar code without the ctype library (no isdigit). It iterates over the characters in the argument and checks if they are digits using this condition:

if (argv[1][i] <= '0' || argv[1][i] >= '9')

This is wrong: it rejects 0 and 9 too. But check50 doesn't complain. Suggestion: write test cases using 0 or 9 as key.