There is a problem with the implementation of the [bug report] dialog component destroy-on-close

### Element UI version
2.14.1

### OS/Browsers version
mac chrome

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/CooberDing/pen/VwKydGN?editors=1111

### Steps to reproduce
打开dialog，然后关闭dialog，注意观察控制台的log。
我根据源码发现，destroy-on-close的实现方式是 "this.key ++ "。相当于把"<div :key="key"></div>"这个dom销毁了又重新创建了，导致我dialog内的组件的mounted声明周期在关闭dialog时被调用了，很诡异。

### What is Expected?
dialog内节点被销毁

### What is actually happening?
节点被销毁后又被创建出来