Fix MPD volume command.

The command should return -1 when the volume is not known.

mopidy/frontends/mpd/protocol/status.py

@@ -137,7 +137,7 @@ def status(context):
 
         Reports the current status of the player and the volume level.
 
-        - ``volume``: 0-100
+        - ``volume``: 0-100 or -1
         - ``repeat``: 0 or 1
         - ``single``: 0 or 1
         - ``consume``: 0 or 1
@@ -267,7 +267,7 @@ def _status_volume(futures):
     if volume is not None:
         return volume
     else:
-        return 0
+        return -1
 
 def _status_xfade(futures):
     return 0 # Not supported

tests/frontends/mpd/status_test.py

@@ -42,10 +42,10 @@ class StatusHandlerTest(unittest.TestCase):
         self.assert_('playtime' in result)
         self.assert_(int(result['playtime']) >= 0)
 
-    def test_status_method_contains_volume_which_defaults_to_0(self):
+    def test_status_method_contains_volume_with_na_value(self):
         result = dict(status.status(self.context))
         self.assert_('volume' in result)
-        self.assertEqual(int(result['volume']), 0)
+        self.assertEqual(int(result['volume']), -1)
 
     def test_status_method_contains_volume(self):
         self.mixer.volume = 17