Unstable sorting of [bug report] table

### Element UI version
2.13.0

### OS/Browsers version
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.88 Safari/537.36

### Vue version
2.6.11

### What is Expected?
```plain
# A1, A2 为同一值，只是在列表中顺序前后关系
不排序 A1 A2
升序 A1 A2
倒序 A1 A2
```

### What is actually happening?
```plain
不排序 A1 A2
升序 A1 A2
倒序 A2 A1
```

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ps: 不能多列排序吗？