[Bug Report] 分页器操作，有些奇怪问题，建议更改（Pager operation, some strange problems, suggest to change）

### Element UI version
2.13.2

### OS/Browsers version
window 10 / Google Chrome 84.0.4147.135

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/xiaofan9/pen/LYNqvYy?editable=true%3Dhttps%3A%2F%2Felement.eleme.cn%2F

### Steps to reproduce
切换到第4页，然后换页面大小为100。

Switch to page 4, and then change the page size to 100.

### What is Expected?
能否只触发size-change 里面的参数，给size、current两个值。

Can you only trigger the parameters in size-change and give two values of page-size and current.

### What is actually happening?
此时页面会按顺序触发size-change、current-change 自定义函数，导致页面发起多次请求，也导致数据不一定准确。

At this time, the page will trigger the size-change and current-change custom functions in order, causing the page to initiate multiple requests and the data may not be accurate.