Fix handling default timeout values
    
    This allows overriding defaults with ``None``, i.e. selectively opting
    to never time-out a request. It involves using a customized session
    class which has a notion of overriding a default value with ``None``,
    requests uses it ambiguously for both defaults and "never time-out".
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>