Add new argument $options in stripe_api_call

Hello.

Stripe library in some methods takes an two arguments: $params and $options. Using function stripe_api_call it's possible to pass only $params, because we calling methods from Stripe library classes using this code:

call_user_func(array($class, $method), $params);

It would be nice if we change the function and add new argument $options.
This argument allows you to add additional headers to the request.