Position freezes after seek in FLAC via HTTP
Mopidy 2.0 9c05cc8d0581a5b4edfdf9ce069c8b358701c4d7. To reproduce:
- play FLAC audio from UPnP server (or internet)
- seek
- -> position freezes

Problem: core.PlaybackController._on_position_changed is called with 0 instead of the expected position 171150, so seek never finishes. 

https://github.com/mopidy/mopidy/blob/develop/mopidy/core/playback.py#L233

```
INFO     seek pending_pos=171150
INFO     Audio event: position_changed(position=0L)
INFO     _on_position_changed pos=0 pen_pos=171150
```

diff --git a/mopidy/core/playback.py b/mopidy/core/playback.py
index ab96171e..da505b22 100644
--- a/mopidy/core/playback.py
+++ b/mopidy/core/playback.py
@@ -230,8 +230,8 @@ class PlaybackController(object):
                 self._seek(self._pending_position)
 
     def _on_position_changed(self, position):
-        if self._pending_position == position:
-            self._trigger_seeked(position)
+        if self._pending_position is not None:
+            self._trigger_seeked(self._pending_position)
             self._pending_position = None
 
     def _on_about_to_finish_callback(self):
diff --git a/tests/core/test_playback.py b/tests/core/test_playback.py
index 3572800c..34c9d367 100644
--- a/tests/core/test_playback.py
+++ b/tests/core/test_playback.py
@@ -734,6 +734,7 @@ class EventEmissionTest(BaseTest):
 
         self.core.playback.play(tl_tracks[0])
         self.trigger_about_to_finish(replay_until='stream_changed')
+        self.replay_events()
         listener_mock.reset_mock()
 
         self.core.playback.seek(1000)
['tests/core/test_playback.py', 'mopidy/core/playback.py']