- When signing multiple documents (if 'Allow only one document' is
  disable) then 'Save as attachment' is not applied and signed result is
  not saved as attachment.
- Add tests.
- Why not taking the occasion to add the whole configuration at report
  level (if to be signed or not, the domain, etc...)? See
  <https://github.com/OCA/reporting-engine/pull/533#issuecomment-898321161>
