[Bug Report] Alignment of Selection Column in Table Header

### Element UI version
2.13.0

### OS/Browsers version
Win10 / Chrome (current)

### Vue version
2.5.13

### Reproduction Link
https://jsfiddle.net/glittle/o19wku35/3/

### Steps to reproduce
See jsfiddle.

### What is Expected?
The checkbox in the header should be aligned with the ones below it.

### What is actually happening?
The checkbox is shifted to the left.
The CSS aligning the box is being overridden by other CSS.