message: balance_transaction is optional when capture = false
  balance_transaction is optional when capture = false
diff: @@ -161,7 +161,7 @@ pub struct Charge {
  pub amount_refunded: u64,
  pub application: Option<String>,
  pub application_fee: Option<String>,
  - pub balance_transaction: String,
  + pub balance_transaction: Option<String>,
  pub captured: bool,
  pub created: Timestamp,
  pub currency: Currency,
  