more robust WebSocket close detection in the sync client (Fixes #346)

src/engineio/client.py

@@ -494,7 +494,7 @@ def _read_loop_websocket(self):
             p = None
             try:
                 p = self.ws.recv()
-                if len(p) == 0:  # pragma: no cover
+                if len(p) == 0 and not self.ws.connected:  # pragma: no cover
                     # websocket client can return an empty string after close
                     raise websocket.WebSocketConnectionClosedException()
             except websocket.WebSocketTimeoutException: