luhn: add a test case of strings with non-digits (#1246)

Author: idealhack

exercises/luhn/canonical-data.json

@@ -1,6 +1,6 @@
 {
   "exercise": "luhn",
-  "version": "1.1.0",
+  "version": "1.2.0",
   "cases": [
     {
       "description": "single digit strings can not be valid",
@@ -105,6 +105,14 @@
         "value": "091"
       },
       "expected": true
+    },
+    {
+      "description": "strings with non-digits is invalid",
+      "property": "valid",
+      "input": {
+        "value": ":9"
+      },
+      "expected": false
     }
   ]
 }