Feature Request: Attach a PaymentMethod to a Customer

[phayes]

I've found that I'm needing this API:

https://stripe.com/docs/api/payment_methods/attach

It would be nice if it was implemented.

Thanks for the awesome crate!

[phayes]

This is the workaround I have for now:

pub fn attach_payment_method(
    client: &stripe::Client,
    customer_id: String,
    payment_method: stripe::PaymentMethodId,
) -> stripe::Response<stripe::PaymentMethod> {
    #[derive(Serialize)]
    struct AttachPaymentMethod {
        customer: String,
    }
    let params = AttachPaymentMethod {
        customer: customer_id,
    };
    client.post_form(
        &format!("/payment_methods/{}/attach", payment_method),
        params,
    )
}