call reimbursement with id method

When using Resque or Sidekiq for background jobs, its best practice to simply introduce a string or an integer instead of an object.

Throughout spree's models, one will find that the emails use the id to fetch the data instead of passing the full object.

Here, I pass the id method into the reimbursement email to ensure that the `Spree::ReimbursementMailer` is compliant with the Resque and Sidekiq best practices.