message: Make deprecated `closed` field optional.
  The field has been removed since
  This change makes the field
  optional, allowing compatibilty with api versions before and after the
  date of change, with minor inconvinience.
diff: @@ -139,7 +139,7 @@ pub struct Invoice {
  pub attempt_count: u64,
  pub attempted: bool,
  pub charge: Option<String>,
  - pub closed: bool,
  + pub closed: Option<bool>,
  pub currency: Currency,
  pub customer: String,
  pub date: Timestamp,
  