[Bug Report] Problem with <el-table> not being able to render data, but not sure if it was a bug in ElementUI itself

### Element UI version
2.12.0

### OS/Browsers version
window7/chrome77

### Vue version
2.6.10

### Reproduction Link
https://github.com/flamesnow/question.git

### Steps to reproduce
cnpm i / npm i
cnpm start / npm start

### What is Expected?
I bound a data for , I hoped that could render tableData data, but found that it could not. Miraculously, all components except could render according to the data, such as v-for loop.

### What is actually happening?
binds the data, but it is not rendered. See demo on github for details.