[Bug Report] Date-range picker sometimes can't show the popper correctly when change its content

### Element UI version
2.12.0

### OS/Browsers version
Mac OSX 10.12 / Chrome 76.0.3809.132

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/jerrybendy/pen/eYYBoay?editors=1010

### Steps to reproduce
Please refer to the codepen example.

1. Click `Monthly` button
2. Click the date-range input
3. Click `Daily` button (Note, DO NOT make the date-range popper invisible)
4. Click the date-range input again
5. Repeat step 1 to 4 few times, the bug would appear.

### What is Expected?
The date-range popper should display correctly.

### What is actually happening?
The date-range proper shows in a wrong position.

I checked the DOM in inspector, the date-range popper should be mounted under `body` element. When the bug appears, the popper is mounted under `el-input` element. this courses the popper shows in a incorrect position.