[Bug Report] ElTableColumn class-name prop is not reactive

[Bounty31]

Element UI version

2.4.1

OS/Browsers version

Windows 10 / Chrome V67

Vue version

2.5.16

Reproduction Link

https://jsfiddle.net/n5p4cf6e/2/

Steps to reproduce

Click on the 'Switch column class' button

What is Expected?

The cells should have the class 'new-class' applied on them.

What is actually happening?

The class is not applied, it looks like it only updates the cell's class on first load and is not reactive.