DatePicker: fix dates selection date jump (#10973)

see issue #10890

packages/date-picker/src/panel/date.vue

@@ -188,6 +188,7 @@
       },
 
       value(val) {
+        if (this.selectionMode === 'dates' && this.value) return;
         if (isDate(val)) {
           this.date = new Date(val);
         } else {

packages/date-picker/src/util/index.js

@@ -28,6 +28,7 @@ export const toDate = function(date) {
 export const isDate = function(date) {
   if (date === null || date === undefined) return false;
   if (isNaN(new Date(date).getTime())) return false;
+  if (Array.isArray(date)) return false; // deal with `new Date([ new Date() ]) -> new Date()`
   return true;
 };