Symbol keys are not merged

[tchin25]

Environment

$ node -v
v20.13.1

Reproduction

const [a, b, c] = [Symbol("a"), Symbol("b"), Symbol("c")];
const result = defu({ [a]: "a", [c]: ["a", "b"] }, { [a]: "bbb", [b]: "c", [c]: ["c", "d"] });
expect(result).toEqual({ [a]: "a", [b]: "c", [c]: ["a", "b", "c", "d"] });

AssertionError: expected { [Symbol(a)]: 'bbb', …(2) } to deeply equal { [Symbol(a)]: 'a', …(2) }

- Expected
+ Received

  Object {
-   Symbol(a): "a",
+   Symbol(a): "bbb",
    Symbol(b): "c",
    Symbol(c): Array [
-     "a",
-     "b",
      "c",
      "d",
    ],
  }

Describe the bug

Symbol keys are not iterated over the base object, but the other objects use Object.assign which does copy symbol keys.

Additional context

No response

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