Fix ID token verification (#87)

diff --git a/tests/test__helpers.py b/tests/test__helpers.py
index d413adc..b067faa 100644
--- a/tests/test__helpers.py
+++ b/tests/test__helpers.py
@@ -151,3 +151,19 @@
 
     for case, expected in cases:
         assert _helpers.string_to_scopes(case) == expected
+
+
+def test_padded_urlsafe_b64decode():
+    cases = [
+        ('YQ==', b'a'),
+        ('YQ', b'a'),
+        ('YWE=', b'aa'),
+        ('YWE', b'aa'),
+        ('YWFhYQ==', b'aaaa'),
+        ('YWFhYQ', b'aaaa'),
+        ('YWFhYWE=', b'aaaaa'),
+        ('YWFhYWE', b'aaaaa'),
+    ]
+
+    for case, expected in cases:
+        assert _helpers.padded_urlsafe_b64decode(case) == expected