message: Replace Sub::cancel one-off arg with params struct
diff: @@ -3,6 +3,11 @@ use client::Client;
  use resources::{Discount, Plan};
  use params::{List, Metadata, Timestamp};
  +#[derive(Default, Serialize)]
  +pub struct CancelParams {
  + #[serde(skip_serializing_if = "Option::is_none")] pub at_period_end: Option<bool>,
  +}
  +
  #[derive(Serialize)]
  pub struct ItemParams<'a> {
  pub plan: &'a str,
  @@ -85,8 +90,7 @@ impl Subscription {
  /// Cancels a subscription.
  /// For more details see https://stripe.com/docs/api#cancel_subscription.
  - pub fn cancel(client: &Client, subscription_id: &str, at_period_end: bool) -> Result<Subscription, Error> {
  - let path = format!("/subscriptions/{}/cancel?at_period_end={}", subscription_id, at_period_end);
  - client.post(&path, ())
  + pub fn cancel(client: &Client, subscription_id: &str, params: CancelParams) -> Result<Subscription, Error> {
  + client.post(&format!("/subscriptions/{}/cancel", subscription_id), params)
  }
  }
  