fix: surrogateKeys not being added when using CoreCache

[pablodelolmo]

The runtime function was incorrectly evaluating the vary parameter expecting an iterator, instead of the surrogateKey one. This was making the SDK to throw TypeError: can't convert undefined to object when doing this:

let [writer, reader] = cachedResponse.insertAndStreamBack({
    maxAge: 120000,
    surrogateKeys: ["test"],
});