Update a Sender

**This endpoint allows you to update an existing Sender.** Updates to `from.email` require re-verification. If your domain has been authenticated, a new Sender will auto verify on creation. Otherwise, an email will be sent to the `from.email`. Partial updates are allowed, but fields that are marked as "required" in the `POST` (create) endpoint must not be nil if that field is included in the `PATCH` request.