Correctly process missing refreshable_until token info
    
    If refreshable_until=None, it ends up not getting stored in the token
    information table.
    This will make sure we will correctly interpret that as "no expiry".
    
    Fixes: #264
    Merges: #281
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Randy Barlow <randy@electronsweatshop.com>
    Reviewed-by: Howard Johnson <merlin@merlinthp.org>