fix: allow keys from Object.prototype like 'constructor' etc as value in attribute 'key'

[rx-837]

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:
When i use "constructor" as attribute "key", i have warning in console: - [Vue warn]: Duplicate keys detected: 'constructor'. This may cause an update error.

https://codesandbox.io/s/angry-stitch-kb2go1

It's behavior because of incorrect object creation inside checkDuplicateKeys function.

const seenKeys = {} - mean make object with default prototype(Object.prototype).