fix: isPlainObject incorrectly identifies objects wrapped in a Proxy object in Safari 10

[reed-soul]

Hi there!

I noticed that in Safari 10, the isPlainObject method used in vue-i18n can lead to an issue where objects wrapped in a Proxy object, such as those returned by the ref method in Vue, are incorrectly identified as non-plain objects. This can result in an empty messages object and the inability to retrieve localized content.

To address this issue, I propose changing the isPlainObject method used in vue-i18n to use the Object.prototype.constructor property to check if an object is a plain object. This method is more reliable and performs well in all browsers, including Safari 10.

Here's the updated code for the isPlainObject method:

function isPlainObject(obj) {
  if (typeof obj !== 'object' || obj === null) return false
  const proto = Object.getPrototypeOf(obj)
  return proto === null || proto.constructor === Object
}

This change should resolve the issue in Safari 10 and ensure that objects wrapped in a Proxy object are correctly identified as plain objects.

Please let me know if you have any questions or concerns. Thanks for considering this PR!

Best regards

closes(#1378)

[kazupon]

Sorry for my late reply!
Good contribution!
Thanks!