CaseInsensitiveArray is an array-like class that ignores case for keys.
It is used to store HTTP headers. Per RFC 2616, section 4.2: Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive.
In the context of stripe-php, this is useful because the API will return headers with different case depending on whether HTTP/2 is used or not (with HTTP/2, headers are always in lowercase).