Fix items serialization when items is a dict

[ob-stripe]

r? @dpetrovics-stripe (DP run, feel free to re-assign)
cc @stripe/api-libraries @remi-stripe

Fixes an issue where if you set items to an integer-indexed dict on a subscription that was retrieved from the API, the library would try to unset the properties of the list object that was returned in the items attribute.

This is basically exactly the same problem as the 2nd issue mentioned here: stripe/stripe-ruby#596, and the fix is similar (simply ignore the previous value of items).

[grey-stripe]

I'm fairly nervous about this change, I don't think we should be digging around in the parent class's implementation details, but I'll need to investigate a bit to see if there's a better way to approach this.

[brandur-stripe]

Yeah, it is a little unfortunate that we need to go up and touch the parent's internals.

Just trying to think of some more options: I guess one possibility is to expose a "protected" (in the nomenclature of other OO languages that support more OO stuff) API on the parent that's designed to be called from a child for exactly this purpose (e.g. unset_previous or something).

[ob-stripe]

Closing this super old PR. The issue was probably fixed by #473, though the current state of the library makes it difficult to write a test for this (since the encoding happens in the api_requestor class, which we mock in tests).