Code review comment for lp:~brunonova/software-properties/lp1381050_2

I've tested the change provided in Wily and it doesn't seem to fix the Traceback for me. Here is a result of trying import the key from the related bug report:

ipdb> n
dbus.exceptions.DBusException: org.freedesktop.DBus.Python.UnicodeEncodeError: Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/dbus/service.py", line 707, in _message_cb
    retval = candidate_method(self, *args, **keywords)
  File "/usr/lib/python3/dist-packages/softwareproperties/dbus/SoftwarePropertiesDBus.py", line 288, in AddKey
    return self.add_key(path)
  File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 804, in add_key
    path = path.encode("utf-8")
  File "/usr/lib/python3.4/genericpath.py", line 19, in exists
    os.stat(path)
UnicodeEncodeError: 'ascii' codec can't encode character '\xed' in position 58: ordinal not in range(128)