Encode arrays as hashes when needed

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries

Fixes #554.

Back in #467, we introduced support for transparently turning the items parameter to a hash of hashes if needed when calling Subscription#create or Subscription#update. However, this failed to handle the case where you'd retrieve a subscription, set the items attribute and save the subscription.

There were actually two different issues going on in the above case:

1. The parameter was not being converted to a Hash if needed.

2. Even if the user provided a Hash directly, the library would try to unset the fields on the existing ListObject of the subscription's items attribute.

This PR fixes both issues, by calling Util.array_to_hash and by passing nil for the original value when serializing items.

It also calls Util.array_to_hash on a few other methods/parameters as needed.

[brandur-stripe]

Nice work OB! Thanks for remembering to tackle this one.