Updated getClientCredentials() to be able get client credentials from Authorization header

And yes @bshaffer I know you don't like it https://github.com/bshaffer/oauth2-server-php/issues/240 but still can we vote for it? :D

It works, but don't merge this. `Response::getHeadersFromServer()` needs to be updated too. Can you say why we need `getHeadersFromServer()`? maybe this is CGI issue or IIS? on apache working fine..

And sorry if I am very wrong.