add example with zero in a string to true_and_false koans

Author: karptonite

python2/koans/about_true_and_false.py

@@ -39,3 +39,4 @@ def test_everything_else_is_treated_as_true(self):
             __,
             self.truth_value("Python is named after Monty Python"))
         self.assertEqual(__, self.truth_value(' '))
+        self.assertEqual(__, self.truth_value('0'))

python3/koans/about_true_and_false.py

@@ -36,3 +36,4 @@ def test_everything_else_is_treated_as_true(self):
         self.assertEqual(__, self.truth_value(1,))
         self.assertEqual(__, self.truth_value("Python is named after Monty Python"))
         self.assertEqual(__, self.truth_value(' '))
+        self.assertEqual(__, self.truth_value('0'))