[Bug Report] Table-column scoped header slots not updating properly

### Element UI version
2.13.2

### OS/Browsers version
macOS Catalina 10.15.4 / Chrome Stable 84.0.4147.125

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/quanyc/pen/vYGXgXZ

### Steps to reproduce
In the demo, a `col` array is used to create multiple ``s with v-for. When "Increase Amount" is clicked, a new array will be assigned to `col`, with new label values and amount values.

### What is Expected?
Column headers should update to reflect the changes.

### What is actually happening?
Column headers didn't update unless we force update them with "Force Update" button. In addition, if we don't assign a new array but modifies the original objects in `col`, it works well, as the third button shows.