bpd: make noidle a no-op outside idle mode

The real MPD ignores `noidle` when the client is not idle. It doesn't
even send a successful response, just ignores the command. Although
I don't understand why a client would fail to keep track of its own
state, it seems that this is necessary to get ncmpcpp working.

beetsplug/bpd/__init__.py

@@ -847,6 +847,9 @@ class MPDConnection(Connection):
                     yield self.send(err.response())
                     break
                 continue
+            if line == u'noidle':
+                # When not in idle, this command sends no response.
+                continue
 
             if clist is not None:
                 # Command list already opened.

test/test_player.py

@@ -457,6 +457,14 @@ class BPDQueryTest(BPDTestHelper):
             response = client.send_command('noidle')
         self._assert_ok(response)
 
+    def test_cmd_noidle_when_not_idle(self):
+        with self.run_bpd() as client:
+            # Manually send a command without reading a response.
+            request = client.serialise_command('noidle')
+            client.sock.sendall(request)
+            response = client.send_command('notacommand')
+        self._assert_failed(response, bpd.ERROR_UNKNOWN)
+
 
 class BPDPlaybackTest(BPDTestHelper):
     test_implements_playback = implements({