Client request to token endpoint does not encode clientKey and secret properly

[hjalet]

In the OpenID Connect Client:
When using a shared secret to request tokens at the token endpoint after a successful login,
The clientKey and Secret does not seem to be encoded correctly.
So a '/' in the secret is correctly encoded to %2F
While '+' and '=' signs in the client secret is not encoded at all.

The encoding takes place in line 248 in OIDCAuthenticationFilter class