luhn: add a test case

[idealhack]

https://github.com/exercism/problem-specifications/blob/master/exercises/luhn/description.md

[rpottsoh]

At a quick glance your addition seems reasonable.
However, please increase second version component of the JSON file. If current version is x . y . z, make the new version x . y+1 . 0

[rpottsoh]

How is this new case different from "valid strings with a non-digit included become invalid"?

[idealhack]

package luhn

import "strings"

func Valid(str string) bool {
	str = strings.Replace(str, " ", "", -1)
	if len(str) < 2 {
		return false
	}

	var sum = 0
	for i, ch := range str {
		val := int(ch - '0')
		if (len(str)-i)%2 == 0 {
			product := val * 2
			if product > 9 {
				product -= 9
			}
			sum += product
		} else {
			sum += val
		}
	}

	return sum%10 == 0
}

The code with a mistake above will pass current tests, but not this one.

So we have to check non-digits by adding:

if val > 9 || val < 0 {
    return false
}

[rpottsoh]

Thanks for the explanation. I can see now how "valid strings with a non-digit included become invalid" passes without the new test case.

Other reviewers may ask that your new test case be moved to an earlier location in the JSON file. I'll leave that open to others to address.

@idealhack thanks for your time in improving this exercise! 👍

[idealhack]

@rpottsoh I've increased the minor version number, please take a look

[rpottsoh]

version looks good. I am going to let this PR sit for now, to allow time for others to weigh in the change you are proposing.

[rpottsoh]

I think enough time has passed for others to convey their thoughts on this PR. I am going to go ahead and merge. Thanks @idealhack for giving some of your time to Exercism and making Luhn a better exercise.

[idealhack]

@rpottsoh You are welcome. Exercism is great because of your work.