fix: deep_merge should stop recursing at class instances

[johnnysprinkles]

In #1662 @JBusillo notes that my #1572 is causing an issue when you pass key and cert values in kit.vite.server.https.

I see that he's passing buffers like this:

const config = {
  kit: {
    // hydrate the <div id="svelte"> element in src/app.html
    target: "#svelte",
    vite: {
      server: {
        https: {
          key: fs.readFileSync("example.key"),
          cert: fs.readFileSync("example.crt"),
        },
      },
    },
  },
};

which my deep_merge function tries to deeply clone. Those Buffer objects have a ton of methods on them. Not sure why this doesn't work (need to look more deeply into it), but it seems like we'd want to not clone those rich object and only clone "plain old JS objects" who's constructor is Object. Things that aren't class instances or built-ins.

This is a quick fix. I need to add tests but I'm on the road, won't get to it until this weekend.

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[johnnysprinkles]

Oh I see what's going on, those two methods toString and toLocaleString, those are enumerable properties of Buffer, but aren't enumerable properties of a plain object. This is an issue I didn't forsee. This fix will fix that though, treating Buffer as opaque.

[johnnysprinkles]

Thanks for merging Ben... note that there is still a minor issue that if you had a config like:

vite: {
  server: {
    toString: () => {}
  }
}

It would still fail due to that conflicting with the Object's own method. I'll have another small fix for that later.