Handle case when adding a new additional owner to the array

[ob-stripe]

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

Fixes #521.

I've verified that without the fix, the new test does trigger the issue reported by @karlr-stripe in #521:

There was 1 error:

1) Stripe\AccountTest::testSerializeAddAdditionalOwners
Undefined offset: 2

/Users/ob/workspace/php/stripe-php/lib/Account.php:235
/Users/ob/workspace/php/stripe-php/lib/Account.php:208
/Users/ob/workspace/php/stripe-php/tests/Stripe/AccountTest.php:225

[brandur-stripe]

LGTM.

[ob-stripe]

Released as 6.17.2.