[Bug Report] How to add prop in slot-scope column

### Element UI version
2.13.0

### OS/Browsers version
10 / chrome

### Vue version
3.0.0-alpha.5

### Reproduction Link
https://jsfiddle.net/mmx38qxw/

### Steps to reproduce
I have a column in my table which uses `slot-scope` but i cannot define `prop` to this column in order to get filters for it.

Code

**filter input**

```html

```

**column in table**

```html

{{ role.name }}

```

**script**

```javascript
filters: [
{
value: [],
prop: 'role',
}
]
```

### What is Expected?

Adding prop to column

### What is actually happening?

Error that I get

```bash
prop role not exist in the row, please confirm wether the prop is right, this may cause unpredictable filter result
```
**screenshot**


**Any idea?**