Array Functions on Objects Soon to be Deprecated

[RealityRipple]

Objects currently behave fairly close to Arrays, however PHP 7.4 will be adding a notice of deprecation for many Array-purpose-functions on Objects. It would probably be a good idea to make a few preemptive changes before 7.4 is released.
So far, I've only noticed something that would be a problem on line 402 of lib/ApiRequestor.php, though there may be other instances. array_key_exists will throw a warning because $rheaders is an Object, not an Array.

[ob-stripe]

Thanks for the heads up @RealityRipple! I had no idea array_key_exists() did not handle \ArrayAccess and just fell back to object properties.

I've opened a PR to start testing PHP 7.4, and sure enough those builds are red, e.g. https://travis-ci.org/stripe/stripe-php/jobs/578611239.

We'll fix this before PHP 7.4 is officially released.

[ob-stripe]

This is the RFC describing the new behavior: https://wiki.php.net/rfc/deprecations_php_7_4#array_key_exists_with_objects

[ob-stripe]

This is fixed in stripe-php v7.0.0.