Revert back to initialize_from from Util.convert_to_stripe_object

[rattrayalex-stripe]

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

In ea736eb, on #790, we changed most uses of initialize_from to be Util.convert_to_stripe_object. We believed this to be equivalent, but this was incorrect; as @AnotherJoSmith pointed out, this returned new objects without mutating instances in-place (or, more precisely, copying over previous attributes).

For example, I believe this would have broken flows like this:

charge = Stripe::Charge.retrieve id
charge.capture
assert charge.captured == true # broken!

I hope to implement a smoother fix to this in a follow-on, but for now, I reverted the offending commit, and tracked down the only new instances of Util.convert_to_stripe_object that have happened since (see this diff and grep `Util.convert_to_stripe_object).

I also hope to add tests shortly, but want to bias to getting this out.

[brandur-stripe]

Thanks Alex. I think a pretty good case could be made that we actually want a new version of the object instead of refreshing the current instance, but big +1 to reverting to what we had previously while working on something else.

LGTM.

[brandur-stripe]

Released as 4.21.2.