遇到 let url = URL(string: str) 转出来的url 结果是nil
解决方法：

var _url : String?
    var url : String {
        get {
            return _url ?? ""
        }
        set (newValue) {
            _url = newValue
            
            //个别网页链接不能转url
            //使用默认编码（通常是 UTF-8）进行百分比编码
            if let escapedString = newValue.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed) {
                let url = URL(string: escapedString)
                webView.load(URLRequest(url: url!))
            }
        }
    }