Default value for socket.gethostname() should be str, not bytes

[PierreQuentel]

https://github.com/mahmoud/boltons/blob/270e974975984f662f998c8f6eb0ebebd964de82/boltons/iterutils.py#L1322-L1332

If the default value is used, '-'.join([...]) fails because the second item is bytes, not str.