add test case from pull request description

Author: stefankoegl

tests.py

@@ -216,6 +216,9 @@ def test_trailing_escape(self):
     def test_invalid_escape(self):
         self.assertRaises(JsonPointerException, JsonPointer, '/foo/bar~2')
 
+    def test_leading_zero(self):
+        doc = [0, 1, 2]
+        self.assertRaises(JsonPointerException, resolve_pointer, doc, '/01')
 
 class ToLastTests(unittest.TestCase):