AuthListener now uses the correct key in the header for HTTP Auth

As documented at https://github.com/gitlabhq/gitlabhq/tree/master/doc/api the key in the header for the token must be "PRIVATE-TOKEN". I don't know, if this has changed compared to earlier versions, where "private_token" might have been correct. But in the current stable version of the API, only "PRIVATE-TOKEN" works.