Fix rpm_hdr_size file closing
    
    The current code will try to close a passed in file object, and will not
    try to close a file object it opened when a string is passed in.
    This results in either a leaked file object, or a file object that was
    passed in to be closed after the function is called.
    
    Signed-off-by: Patrick Uiterwijk <patrick@puiterwijk.org>