fix(auth): ios providerData properties undefined

[delaneyb]

This PR fixes an issue where all properties of providerData values on the user were appearing as undefined on iOS e.g.

console.log(firebase.auth().currentUser.providerData[0].toJSON())

produces

{
  displayName: undefined
  email: undefined
  phoneNumber: undefined
  photoURL: undefined
  providerId: undefined
  uid: undefined
}