[bug report] invalid when canceling single field validation in form validation

### Element UI version
2.15.2

### OS/Browsers version
win10

### Vue version
2.6.13

### Reproduction Link
https://elementui.github.io/issue-generator/#/zh-CN

### Steps to reproduce
查看源码 clearValidate(props = []) {
const fields = props.length
? (typeof props === 'string'
? this.fields.filter(field => props === field.prop)
: this.fields.filter(field => props.indexOf(field.prop) > -1)
) : this.fields;
fields.forEach(field => {
field.clearValidate();
});
}, 此方法中，过滤回调中field对象下没有prop属性，通过vue-devtools查看this.fields数组里面的每一项下面有一个props属性，再props属性下才有prop属性。所以直接返回props === field.prop是无效的。因为prop不在field下面，应该是props === field.props.prop

### What is Expected?
请确定后及时修改

### What is actually happening?
请确定后及时修改