[Bug Report] Table's width of column does Not automatically fits its container in Safari and one line mode

### Element UI version
2.6.1

### OS/Browsers version
macOS 10.14.3; Safari 12.0.3 (14606.4.5)

### Vue version
2.6.9

### Reproduction Link
[https://jsfiddle.net/vqhp4ksg/](https://jsfiddle.net/vqhp4ksg/)

### Steps to reproduce
Please check the jsfiddle above.

Note: the code work correctly in Chrome, but not in Safari.

### What is Expected?
Table's width of column automatically fits its container in Safari and one line mode

### What is actually happening?
The width is dynamic with table content.