Fix results for digit-only and white-space-only input

As the algorithm ignores characters which occur in all or
most languages, `getTopScript` returned `cmn` for these
values (for example, `123 456 789`), as it is the first
script to test and other scripts to reach higher scores.

This bug-fix changes that by checking for a score of `0`, and
returning the `und` string/tuple.

Closes GH-18.

lib/franc.js

@@ -294,10 +294,15 @@ function detectAll(value, options) {
 
     /*
      * One languages exists for the most-used script.
+     *
+     * If no matches occured, such as a digit only string,
+     * exit with `und`.
      */
 
     if (!(script[0] in data)) {
-        return singleLanguageTuples(script[0]);
+        return singleLanguageTuples(
+            script[1] === 0 ? 'und' : script[0]
+        );
     }
 
     /*

test/index.js

@@ -108,6 +108,10 @@ describe('franc()', function () {
 
         assert(result === 'und');
     });
+
+    it('should return `und` for generic characters', function () {
+        assert(franc('987 654 321') === 'und');
+    });
 });
 
 describe('franc.all()', function () {
@@ -143,6 +147,14 @@ describe('franc.all()', function () {
         assert(result.length === 1);
     });
 
+    it('should return `[["und", 1]]` for generic characters', function () {
+        var result = franc.all('987 654 321');
+        assert(result[0][0] === 'und');
+        assert(result[0][1] === 1);
+        assert(result[0].length === 2);
+        assert(result.length === 1);
+    });
+
     it('should work on weird values', function () {
         var result = franc.all('the the the the the ');