fix: handle defaults correctly

[curbengh]

noticed Object.assign() doesn't do deep cloning,

In the following config, the defaults level and collisionSuffix are not parsed to the plugin in current behavior.

markdown:
  anchors:
    permalink: true

This PR ensure defaults are handled correctly, so that level and collisionSuffix are retained.

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also fixed an issue where duplicate titles are not handled correctly, should be Object.prototype.hasOwnProperty(), not Object.prototype.isPrototypeOf()