GDM discovery assumes ASCII

[JshWright]

The GDM discovery component assumes it will be receiving ASCII. This is likely to cause an issue if someone has non-ASCII characters in their Plex server name. @nauticus25 reported the following crash in the Home Assistant gitter channel:

Exception in thread Thread-3:
Traceback (most recent call last):
  File "/usr/lib/python3.4/threading.py", line 920, in _bootstrap_inner
    self.run()
  File "/home/pi/.homeassistant/deps/netdisco/service.py", line 60, in run
    self._scan()
  File "/home/pi/.homeassistant/deps/netdisco/service.py", line 74, in _scan
    self.discovery.scan()
  File "/home/pi/.homeassistant/deps/netdisco/discovery.py", line 57, in scan
    self.gdm.scan()
  File "/home/pi/.homeassistant/deps/netdisco/gdm.py", line 23, in scan
    self.update()
  File "/home/pi/.homeassistant/deps/netdisco/gdm.py", line 89, in update
    data = data.decode('ascii')
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 124: ordinal not in range(128)

My guess is it would be safer to assume utf-8 when decoding, but I don't have any Plex stuff, so I'm not able to test that.