Do not mutate params when encoding arrays as indexed objects

[rattrayalex-stripe]

Fixes #521

Also, for good measure, does a shallow copy of request data in general – I don't think there were any other situations that would have mutated this today, and it also doesn't protect against deep mutations, but I don't think it can hurt much (I wouldn't guess that the extra copy operations would have a meaningful perf impact).

cc @jperasmus
cc @ob-stripe
cc @enugent-stripe

[jlomas-stripe]

lgtm 👍

[jperasmus]

Thanks for the quick fix @rattrayalex-stripe

I see you went for the shallow copy approach. In this situation, it looks to be a good solution because the encodeParamWithIntegerIndexes utility method only applies the arrayToObject transformation one level deep.

For situations with more deeply nested data objects, it will still be copied over by reference, but luckily won't be mutated here.

One example I can think of right now is when you have a customer object with shipping details including the address object.

Something like:

const customer = await stripe.customers.update(customerId, {
  shipping: {
    address: {
      line1: 'Some street',
      // etc
    }
  }
});

[rattrayalex-stripe]

@jperasmus - yep, sorry, should have addressed directly; for now, a shallow copy is fine since we don't (can't, actually, I think) modify deep properties (please correct me if I'm wrong!).

If/when we add support for "encoding" deeply nested values, it'll require a code change – I'll be sure to check that we're more careful with our inputs then 😄