Update docblock on `EndpointGateway::makeApiRequest`.

Hint about `$method` parameter assumed this library was still using the Pecl OAuth library.
Validate subscription type, de-dupe logic in the process of adding validation.

Updated `UserGatway::deleteSubscription`: should support subscriber id header. Also get rid of uninitialized variable.

Added methods: `UserGateway::makeSubscriptionHeaders`, `UserGateway::makeSubscriptionUrl`, and `UserGateway::validateSubscriptionType`.