Decode back possible Base64-encoded state in responseHandler

[ioliva]

In PR #658 the property base64_state was added, to force the state param to be base64-encoded instead of URI encoded.

That makes the state impossible to decode when the flow returns to the calling page, so we detect if the returned state is in base64 and decode it accordingly if so, first of all.

[ioliva]

Hi @MrSwitch , could you kindly please review this? Thanks!