[Bug Report] datePicker close dropdown menu when selecting a date (even if hide-on-click is false)

### Element UI version
2.9.1

### OS/Browsers version
Windows 10 / Chrome version 74.0.3729.169

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/anon/pen/YbbqpN

### Steps to reproduce
Select date in datePicker when that component is nested in the dropdown component (with hide-on-click=false)

### What is Expected?
Dropdown should not close itself (if hide-on-click is false) after selecting date in datePicker

### What is actually happening?
Dropdown close itself after selecting date in datePicker

Start with the reproduction link and verify the datePicker nested inside a dropdown with hide-on-click=false. Trace the date-selection behavior and dropdown close handling; done means selecting a date leaves the dropdown open while preserving normal closing behavior when hide-on-click is enabled.

Written by the indexing model from the issue text.