When the [bug report] object key value contains. The El form item configuration prop reports an error: error: please transfer a valid prop path to form item!

### Element UI version
2.12.0

### OS/Browsers version
MacOS，Chrome最新

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/mmx38qxw/

### Steps to reproduce
1. 使用 el-form 表单，绑定对象 { 'test.test': 'test' }
2. el-form-item 设置 prop 为 'test.test'

### What is Expected?
正常使用，prop通过 xx.xx 检验嵌套属性是个不错的功能，但是看了源码发现貌似没办法满足键值实际上就是 xx.xx 的情况

### What is actually happening?
报错 Error: please transfer a valid prop path to form item!