fix(reactivity): ensure add/set on reactive collections return the proxy (#2534)

fix #2530

packages/reactivity/src/collectionHandlers.ts

@@ -76,11 +76,11 @@ function add(this: SetTypes, value: unknown) {
   const target = toRaw(this)
   const proto = getProto(target)
   const hadKey = proto.has.call(target, value)
-  const result = target.add(value)
+  target.add(value)
   if (!hadKey) {
     trigger(target, TriggerOpTypes.ADD, value, value)
   }
-  return result
+  return this
 }
 
 function set(this: MapTypes, key: unknown, value: unknown) {
@@ -97,13 +97,13 @@ function set(this: MapTypes, key: unknown, value: unknown) {
   }
 
   const oldValue = get.call(target, key)
-  const result = target.set(key, value)
+  target.set(key, value)
   if (!hadKey) {
     trigger(target, TriggerOpTypes.ADD, key, value)
   } else if (hasChanged(value, oldValue)) {
     trigger(target, TriggerOpTypes.SET, key, value, oldValue)
   }
-  return result
+  return this
 }
 
 function deleteEntry(this: CollectionTypes, key: unknown) {