Ensure that reaching end resets current_track

mopidy/backends/__init__.py

@@ -479,6 +479,7 @@ class BasePlaybackController(object):
             self.state = self.PLAYING
         elif self.next_track is None:
             self.stop()
+            self.current_track = None
 
         # FIXME handle in play aswell?
         if self.consume:

tests/backends/base.py

@@ -814,4 +814,3 @@ class BasePlaybackControllerTest(object):
             self.assert_(self.playback.current_track not in played)
             played.append(self.playback.current_track)
             self.playback.next()
-        self.assertEqual(self.playback.next_track, None)