fix(reactivity): should delete observe value (#598)

fix #597

packages/reactivity/__tests__/collections/WeakSet.spec.ts

@@ -23,6 +23,19 @@ describe('reactivity/collections', () => {
       expect(dummy).toBe(false)
     })
 
+    it('should observe mutations with observed value', () => {
+      let dummy
+      const value = reactive({})
+      const set = reactive(new WeakSet())
+      effect(() => (dummy = set.has(value)))
+
+      expect(dummy).toBe(false)
+      set.add(value)
+      expect(dummy).toBe(true)
+      set.delete(value)
+      expect(dummy).toBe(false)
+    })
+
     it('should not observe custom property mutations', () => {
       let dummy
       const set: any = reactive(new WeakSet())