Allow CaseInsensitiveArray to be traversed

[pbowyer]

Stripe's exceptions return a lot of extra information about the HTTP request, which is awesome when debugging problems.

The HTTP Headers are held in a Stripe\Util\CaseInsensitiveArray collection. This makes it easy to get individual headers, but there's no way to get all of them, either via a ->all() method or by iterating:

foreach ($exception->getHttpHeaders() as $k => $v) {
   // This is never entered
}

When handling the exception I would like to record all headers, so getting them would be very handy.

[ob-stripe]

Hi @pbowyer, thanks for the suggestion! We just released stripe-php 7.1.0. CaseInsensitiveArray is now countable and traversable.

[pbowyer]

Brilliant, thank you!