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

Created by: rx-837

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
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Does this PR introduce a breaking change? (check one)

• Yes
• No

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• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

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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).