rpcserver.login_x509: Actually return reply from __call__ method
    
    __call__ didn't return causing internal error in wsgi application. Previously
    this bug was hidden by some other error and the code worked even though it
    shouldn't.
    
    https://pagure.io/freeipa/issue/6819
    
    Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>