Rename fake magic methods and rewrite array conversion

[ob-stripe]

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

• Rename the "fake" magic methods __toArray and __toJSON to toArray and toJSON. They weren't actually magic methods so the double underscores were misleading and causing warnings (cf. PHP 7.2 Warnings #587).
• Rewrite toArray to no longer rely on Util::convertStripeObjectToArray. The implementation is a bit hard to read but it's basically an exact port of the stripe-ruby logic.
  • Also, toArray no longer accepts a $recursive argument and always converts recursively.
• Remove Util::convertStripeObjectToArray. The method was misleading because it pretended to accept a StripeObject when really it expected the $_values property of a StripeObject. This lead to user confusion (cf. Util::convertToStripeObject and Util::convertStripeObjectToArray don't seem to round-trip #702).

Fixes #587, fixes #702.

[brandur-stripe]

Nice cleanup! LGTM.