`shallowReactive` will unwrap `refs`

### Version
2.7.0

### Reproduction link
[codesandbox.io](https://codesandbox.io/s/vue-v2-7-shallowreactive-bx2u4t?file=/src/index.js)







### Steps to reproduce
See the reproduction link above

### What is expected?
Does not unwrap `refs`

### What is actually happening?
Does unwrap `refs`