fix #8093: Use custom toString method when interpolating objects if implemented

[mathieutu]

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

Description:

Hi,
When interpolating an object, it uses the JSON.stringify method instead of toString, which is normally the proper method to set a way to convert an object to a string.
This PR allow you to use your custom toString method to render your object if it exists. It's tested (I've also added a test for toJson), and it should not be a breaking change.

Thanks you for your amazing work folks, and see you in Paris next month!

[mathieutu]

I'm adding some example of the problem to make me understood better:

@posva:

I can confirm that your (so used!) Object.create(null) example works exactly the same than before (like arrays), as it's not a "plain object" (_toString(Object.create(null)) gives [object Undefined]).

And to write here what I told you yesterday, it could be a BC for those who had a object with a toString method implemented and still want to use the toJson in templates. I think we can easily consider that this is not a standard case. 😅

See you! 👋

[phanan]

I think this function can really use early returns now.

[mathieutu]

As you want, I kept the original style to make as few changes as possible and let the diff easier to read, but it'll be easy to refactor.