Fix base64 encoding

- Drop the base64url code
- Escape things ourselves, without dropping the b64 padding
- Modify one of the tests (the key
  'cw_0x689RpI-jtRR7oE8h_eQsKImvJapLeSbXpwF4e4' fails to work as b64
  when trying it with various decoders whereas
  'cw_0x689RpI-jtRR7oE8h_eQsKImvJapLeSbXpwF4e4=' succeeds. Maybe the
  test was wrong?)