[bug report] problems of El select component when using filter method attribute to customize filtering method

### Element UI version
2.14.1

### OS/Browsers version
mac

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/code-farmer-i/pen/vYXZLZB

### Steps to reproduce
1、输入框输入11111搜索
2、使下拉框失去焦点，下拉框收起
3、点击下拉框，使下拉框展开

### What is Expected?
重新展开时，query被重置为了空字符串，触发filter-method，展示出所有选项。

### What is actually happening?
重新展开时，query被重置为了空字符串，却没有触发filter-method，导致下拉框展示无数据。