Changes from 1/11/2014.
Changlog:
2014.11.01:
- Updated "RTSPClient" so that it reuses "fCurrentAuthenticator" if we previously
  updated it with data from a "WWW-Authenticate:" response, even if a non_NULL
  "authenticator" parameter was passed as a parameter to the command.  This reduces the
  number of authetication exchanges that take place if the server asks for authentication
  on more than one command in a RTSP session.
  (Thanks to Tomasz Pala for this suggestion.)
- Updated "DigestAuthenticator" to allow for the possibility of "username" or "password"
  being NULL.
- Updated the "RTSPServer" implementation to add an access check before the first "SETUP"
  (the one that doesn't include a session id), because it's possible, in principle, for a client
  to send such a "SETUP" without first sending a "DESCRIBE".  Therefore, we need to perform
  access checks on both commands.