Implement callback function to convert to JSON

[ThomasG77]

The expected websocket response is JSON. I received some data from a third party websocket that does not use JSON but I want to convert it before.

Changing
https://github.com/nathantsoi/vue-native-websocket/blob/master/src/Observer.js#L33-L36

    if (this.format === 'json') {
      let transformFunction = function transformFunction(content) {
        return content
      };
      if ('transformFunction' in this) {
        transformFunction = this.transformFunction
      }
      if (!('sendObj' in this.WebSocket)) {
        this.WebSocket.sendObj = (obj) => this.WebSocket.send(JSON.stringify(transformFunction(obj)))
      }
    }

Tell me what you think about the proposition