fix daylight saving time close#262 close#329

* fix(dayjs.add): fix daylight saving time

It's not safe to add 24 hours to manipulate date, There are 2 days of the year that do not have 24 hours
In order to get rid of this issue its good to use build-in `setDate`
See: https://en.wikipedia.org/wiki/Daylight_saving_time

src/index.js

@@ -238,12 +238,23 @@ class Dayjs {
       const date = this.set(C.DATE, 1).set(u, n + number)
       return date.set(C.DATE, Math.min(this.$D, date.daysInMonth()))
     }
+    const instanceFactorySet = (n) => {
+      const date = new Date(this.$d)
+      date.setDate(date.getDate() + (n * number))
+      return wrapper(date, this)
+    }
     if (unit === C.M) {
       return instanceFactory(C.M, this.$M)
     }
     if (unit === C.Y) {
       return instanceFactory(C.Y, this.$y)
     }
+    if (unit === C.D) {
+      return instanceFactorySet(1)
+    }
+    if (unit === C.W) {
+      return instanceFactorySet(7)
+    }
     let step
     switch (unit) {
       case C.MIN:
@@ -252,12 +263,6 @@ class Dayjs {
       case C.H:
         step = C.MILLISECONDS_A_HOUR
         break
-      case C.D:
-        step = C.MILLISECONDS_A_DAY
-        break
-      case C.W:
-        step = C.MILLISECONDS_A_WEEK
-        break
       case C.S:
         step = C.MILLISECONDS_A_SECOND
         break