tests: Fix typo in mock usage

The error was made evident by a newer mock version that no longer
swallowed the wrong assert as regular use of a spec-less mock.

tests/mpd/protocol/test_connection.py

@@ -10,7 +10,7 @@ class ConnectionHandlerTest(protocol.BaseTestCase):
     def test_close_closes_the_client_connection(self):
         with patch.object(self.session, 'close') as close_mock:
             self.send_request('close')
-            close_mock.assertEqualResponsecalled_once_with()
+            close_mock.assert_called_once_with()
         self.assertEqualResponse('OK')
 
     def test_empty_request(self):