fix(reactivity): Map/Set identity methods should work even if raw value contains reactive entries

fix #799

packages/reactivity/src/baseHandlers.ts

@@ -23,7 +23,14 @@ const arrayInstrumentations: Record<string, Function> = {}
     for (let i = 0, l = (this as any).length; i < l; i++) {
       track(arr, TrackOpTypes.GET, i + '')
     }
-    return arr[key](...args.map(toRaw))
+    // we run the method using the orignal args first (which may be reactive)
+    const res = arr[key](...args)
+    if (res === -1 || res === false) {
+      // if that didn't work, run it again using raw values.
+      return arr[key](...args.map(toRaw))
+    } else {
+      return res
+    }
   }
 })