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

fix #597

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

@@ -26,6 +26,22 @@ describe('reactivity/collections', () => {
       expect(dummy).toBe(undefined)
     })
 
+    it('should observe mutations with observed value as key', () => {
+      let dummy
+      const key = reactive({})
+      const value = reactive({})
+      const map = reactive(new Map())
+      effect(() => {
+        dummy = map.get(key)
+      })
+
+      expect(dummy).toBe(undefined)
+      map.set(key, value)
+      expect(dummy).toBe(value)
+      map.delete(key)
+      expect(dummy).toBe(undefined)
+    })
+
     it('should observe size mutations', () => {
       let dummy
       const map = reactive(new Map())