Force the padded string to be string

[SanderVerkuil]

The padded string was an unicode string which is not allowed in the base64.urlsafe_b64decode function.

See https://stackoverflow.com/questions/20849805/python-hmac-typeerror-character-mapping-must-return-integer-none-or-unicode/20862445 for reference.

Fixes #31

[phihag]

We are running into the same issue. @DCKramer: any update?