[Bug Report] multiple-select has show label but show id when remote search

### Element UI version
2.12.0

### OS/Browsers version
macOs 10.14.6 / Google Chrome 76.0.3809.132

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/zaoyu63/pen/WNeXLMe

### Steps to reproduce
1, input 'b', select ‘beijing’
2, remove 'b', input 's', select 'shanghai'

### What is Expected?
first select item 'beijing', show 'beijing', not '0'

### What is actually happening?
first select item 'beijing', show '0'