DatePicker : type="monthrange" 关闭选择面板选择状态不重置的bug

[leekbillow]

Please make sure these boxes are checked before submitting your PR, thank you!

• Make sure you follow Element's contributing guide (中文 | English | Español | Français).
• Make sure you are merging your commits to dev branch.
• Add some descriptions and refer relative issues for you PR.

月范围选择面板中选择了一项后，点击其他区域失焦关闭面板，再打开时已选择一项的状态并没有重置：

如果想要限制月范围选择的话，会出现一些难绷的情况：

日范围选择type=daterange没有这个问题，直接把代码CV过来了：if (this.minDate && this.maxDate == null) this.rangeState.selecting = false;
虽然我觉得没有必要if判断，直接设为false就完事了

[leekbillow]

鉴于项目800年都不更新，临时解决方式是在@blur事件中自己手动重置状态。

<template>
  <el-date-picker type="monthrange" @blur="onBlur" ref="datePicker"/>
</template>

<script>
// ......
onBlur() {
      const picker = this.$refs.datePicker.picker;
      if (picker) picker.rangeState.selecting = false;
}
</script>