Must clear the isauth cache when resetting authentication parameter.

This fixes the "User is not authorized for IMP" issue when accessing
IMP data via RPC, and using an auth backend other than Application.
Since RPC first initializes Horde with authentication => none,
when the notification system is initialized, Horde_Registry::_cache['isauth']
is populated when no authentication is present. Further on in the script,
after proper authentication is made, Horde_Registry::isAuthenticated will fail
since it still contains the results of the authentication attempt when authentication
was not available.