fix(reactive): avoid unneccessary reaction

[voderl]

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What have you changed?

如果 object 上的某个 key 已经生成 observableResult，get 的值应该就是 observableResult，但是因为没有 set，内部对象值还是 plainobject，在 set 自己时会额外 reaction 一次（某些情况下会导致 react 额外渲染一次）

避免不必要的 reaction

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