[Bug Report] element-ui DatePicker Picker Options disabledDate

### Element UI version
2.13.0

### OS/Browsers version
MacOS 10.15.2/Google Chrome 79

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/wzc0x0/pen/QWwxGjO

### Steps to reproduce
点击控件，默认一月份禁止选择，虽然禁止选择了，然而禁止的样式丢失了

### What is Expected?
月份禁止样式出现

### What is actually happening?
月份禁止样式没有

Start with the linked CodePen reproduction and inspect the DatePicker month view with the disabledDate option. Reproduce the January case in Chrome, then trace the component behavior responsible for month selection and styling. Done means disabled months both remain unselectable and display their disabled style.

Written by the indexing model from the issue text.