Fix WebSockets prefix matching

[SaltFish001]

I config a wsProxy like this

mediaWS: {
      proxyType: 'websocket',
      target: `ws://${Env.MEDIA_SERVER || 'localhost:3009'}`,
      prefix: `/rtp`,
    },

but when I contenct like this

ws://192.168.135.2:4000/rtp/50010600001180000001_50010600001320000001.live.flv

It does not match this route, then I read the source code. I found the reason in here
in line 18-20

  const prefix = url.pathname || '/' 
  // my prefix get **/rtp/50010600001180000001_50010600001320000001.live.flv**
  // but I want **/rtp**

      if (prefix2route.has(prefix)) {  // maybe can use String.startWith instace?