fix RevokeAuthToken method to work correctly

An app consuming this SDK was failing to properly logout users, investigated and found the RevokeAuthToken method was not actually passing along the refresh token on its HttpRequest to the server... turns out it was missing the '?' in front of the query string so the extension method to parse query strings was failing to pick up the refresh token query parameter.  Made changes locally and tested with my app, logout works as expected with these changes.