fix(firestore): iOS nested FieldValues not serialized in update()

[delaneyb]

This PR resolves the following issue:

Performing an update with a FIeldValue used as a nested property value such as:

await firebase.firestore().doc('test/1234').update({
    a: {
        b: FieldValue.serverTimestamp()
    }
})

Results in the following firestore document contents:

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