luhn: non-digit at end is invalid

[bitfield]

From luhn: add test case with non-digit at the end:

Problem: I had a student only include the digits before the first invalid character. There was no test that failed his solution.

Solution: Added a test that should return false but does not if the student takes only the numbers up to first non-digit.

[rpottsoh]

Let me know if you need me to merge.