Use urlsafe_b64decode instead of b64decode (#84)

* Use urlsafe_b64decode instead of b64decode

* Fix decoding error on Python 2.7

Author: kit494way

msal/oauth2cli/oidc.py

@@ -8,7 +8,10 @@
 def base64decode(raw):
     """A helper can handle a padding-less raw input"""
     raw += '=' * (-len(raw) % 4)  # https://stackoverflow.com/a/32517907/728675
-    return base64.b64decode(raw).decode("utf-8")
+    # On Python 2.7, argument of urlsafe_b64decode must be str, not unicode.
+    # This is not required on Python 3.
+    raw = str(raw)
+    return base64.urlsafe_b64decode(raw).decode("utf-8")
 
 
 def decode_id_token(id_token, client_id=None, issuer=None, nonce=None, now=None):