Fix issue #206

[AndreasBilke]

Use the same logic for confidential client authentication as in token
request with authorization grant.

List of common tasks a pull request require complete

• Changelog entry is added or the pull request don't alter library's functionality

[rosinisi]

I found this very same issue that applies also to refreshToken. I might do a similar pull request for that method.

Edit: found already a pull request that concern this: https://github.com/jumbojett/OpenID-Connect-PHP/pull/215/files

[azmeuk]

Fixed by #271. Sorry I did not see this patch before, as it is older I think this is the one that should have been merged.