Fix crash in NadMixer when using unicode strings for settings

docs/changes.rst

@@ -71,6 +71,11 @@ No description yet.
     any help from the original MPD server.
   - Support UTF-8 encoded tag caches with non-ASCII characters.
 
+- Mixers:
+
+  - Support use of unicode strings for :mod:`mopidy.mixers.nad` specific
+    settings.
+
 - Models:
 
   - Rename and generalize ``Playlist._with(**kwargs)`` to

mopidy/mixers/nad.py

@@ -147,6 +147,8 @@ class NadTalker(BaseThread):
         return self._readline().replace('%s=' % key, '')
 
     def _command_device(self, key, value):
+        if type(value) == unicode:
+            value = value.encode('utf-8')
         self._write('%s=%s' % (key, value))
         self._readline()