Have special character encoding be optional

[2373655r]

Problem

I have an OAuth2 client secret with special characters (=,-,/). Special characters are getting encoded following the change in #594 meaning I cannot authenticate when on 1.3.0 . I also cannot change the client that is generating the secrets containing special characters.

Suggested solution

A parameter that lets you optionally disable encoding special characters in client_id and client_secret, either with the understanding that using a colon character will cause a failure, or have it so that only colon characters are encoded.