fix: correct the has implementation in the _renderProxy

[dcherman]

It's feasible that someone might ask if something other than a string is
in the proxy such as a Symbol that lacks a charAt method. This aligns
the implementation with the getHandler.

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:

This issue was discovered when using vue-test-utils and the associated test helpers it provides. Ref: vuejs/vue-test-utils#452

[dcherman]

Not sure if there's a better way to test this logic other than how I wrote it unless we're willing to access the _renderProxy directly to just run an in test, but that's technically private implementation here. I didn't see any other tests using private impl, so I avoided doing so.