[Bug Report] Pagination component: after switching page size from a smaller number to larger number, the current page not display the right number

### Element UI version
2.13.2

### OS/Browsers version
任意版本(代码逻辑错误)

### Vue version
3.0.0-beta.14

### Reproduction Link
https://codepen.io/pen?__cf_chl_jschl_tk__=d9c72485a594fb846847a7899b000afbca205ea4-1591317425-0-ARGBHlv-kC_hxzuoGg37kwjlXTz5vauAFpFVpQaDSX8_GU90bFoZkoIwfHLQtgm8EQjbIZQv7tV97PnTeQNClxZj_1NRggUozaLOWSI3n5Pl2cMUQt233KjTvtgnNhE6ELEZM4LhrYPRbg_dTVmGdi9N3W27JlN6dSopug5wuF9v6dIXSpV3wNuYmAl64PXgvOFwJi4S8MzSsmKrmOvrPYAkLgkNwvxFbIUvQRMqlNnJCQqvL1BPjIalcoWC7uwMEVN047bPacz_VLNM30gmZptHA3oxXQ46TU_he6f0-S4_1Omvc8bNxyp9YlzOipLrlAFsg09dbctt9AVjh5KZV_k

### Steps to reproduce
Assume you have 1000 records
1.select page size = 100
2. select current page number = 4
3. switch page size to 200

### What is Expected?
current page = 2

### What is actually happening?
current page = 4