Fixed client_id and client_secret retrieval from request

As the [RFC6749](http://tools.ietf.org/html/rfc6749) states in its [2.3.1](http://tools.ietf.org/html/rfc6749#section-2.3.1) section:

>  Clients in possession of a client password **MAY** use the HTTP Basic authentication scheme as defined in RFC2617 to authenticate with the authorization server

> The authorization server **MUST** support the HTTP Basic authentication scheme for authenticating clients that were issued a client password.

I've added this usecase in the AuthServer::getParam() method.