[Bug Report] Dialog contents not destroyed even with :destroy-on-close="true"

### Element UI version
2.13.0

### OS/Browsers version
Chrome Version 79.0.3945.79

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/hollowcast/pen/MWYbQBw

### Steps to reproduce
Create a dialog with option :destroy-on-close="true"

### What is Expected?
Contents of the dialog should be destroyed after it closes. (Similar to the behavior of drawer destroy on close)

### What is actually happening?
Contents are not destroyed.