The issue with a fixed vocabulary

Hi,

I've modified the example test_microphone.py by adding a fixed vocabulary with just one word 'start':
```
.....
vocabulary = '["start", "[unk]"]'
.....
rec = KaldiRecognizer(model, args.samplerate, vocabulary)
.....
```
It works fine when I say 'start'. But it shows 'start' even when I say 'stop'. Am I missing anything here?

Thank you!