try to parse json ;-)

I spent some time to understand why I got

```
kaldi_1 | Error in connection handler
kaldi_1 | Traceback (most recent call last):
kaldi_1 | File "/usr/lib/python3/dist-packages/websockets/server.py", line 169, in handler
kaldi_1 | yield from self.ws_handler(self, path)
kaldi_1 | File "./asr_server.py", line 57, in recognize
kaldi_1 | response, stop = await loop.run_in_executor(pool, process_chunk, rec, message)
kaldi_1 | File "/usr/lib/python3.7/concurrent/futures/thread.py", line 57, in run
kaldi_1 | result = self.fn(*self.args, **self.kwargs)
kaldi_1 | File "./asr_server.py", line 26, in process_chunk
kaldi_1 | elif rec.AcceptWaveform(message):
kaldi_1 | File "/root/.local/lib/python3.7/site-packages/vosk/vosk.py", line 115, in AcceptWaveform
kaldi_1 | return _vosk.KaldiRecognizer_AcceptWaveform(self, data)
kaldi_1 | TypeError: in method 'KaldiRecognizer_AcceptWaveform', argument 2 of type '(const char *data, int len)'
```
and I found my error

```js
ws.send(JSON.stringify({ eof: 1 });
```
'{"eof":1}' !== '{"eof" : 1}'

https://github.com/alphacep/vosk-server/blob/e2b4242251981501e049f918ffda11360ace64a8/websocket/asr_server.py#L44