fix(reactivity): when setting a property the shallowReactive object s…

[dupfei]

…hould not unwrap refs and the reactive object should retain readonly proxy

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[yyx990803]

Can you add relevant test cases for the bugs you are fixing?

[yyx990803]

Thanks for the PR - as I was writing the test case for it, I discovered some edge cases that wasn't even working correctly in Vue 3:

1. Setting a readonly object to a ref nested in a reactive object
2. Setting a readonly/shallow object as ref value

These are fixed for Vue 3 in vuejs/core@a95554d

This PR also has subtle issues that are better explained via test cases - I also took the opportunity to align the behavior with Vue 3, see 2af751b.

[dupfei]

Sorry, I just saw it. Is the property of the reactive object always a ref once it is assigned to a ref, just hidden by the internal unwrap ref behavior?