fix(traverse): Prevent traversing Vue instances

[SergioCrisostomo]

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

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:

This fixes a bug first discovered in Vuex where storing Vue instances in the Store generates a RangeError: Maximum call stack size exceeded.

Related: vuejs/vuex#1587

If there is another approach for this I'm happy to go that road and suggest an alternative fix or documentation improvement..

[posva]

I would discourage you from storing Vue instances in the store. It's better for the store to be fully serializable. I think we have already discussed this but couldn't find the discussion in vue repository.

[SergioCrisostomo]

@posva I thought also this should be named in the docs somewhere. So if this PR leads to that new "good practise rule" I am happy also. It should be clear so developers know if its something we can use or not.

[posva]

Yeah, I understand, I read the issue you posted on Vuex
It is stated in Vue.js Docs: https://vuejs.org/v2/api/#data

The Vuex state property follow the same reactivity rules as Vue data