[Bug Report] Tabs: nav button "is-disabled" class state is incorrect when tabs can scorll

### Element UI version
2.14.1

### OS/Browsers version
windows/chrome

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/privatenine/pen/poExOQR

### Steps to reproduce
when click nav until nav is disabled, but "is-disabled" class is not suit with state

### What is Expected?
when click nav until nav is disabled, "is-disabled" class is correct

### What is actually happening?
when click nav until nav is disabled, "is-disabled" class is incorrect