Fix: allow 'contract_line' with mobile '_is_pack' method
CHANGELOG:
### Fixed
- [#958](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/958) Allow 'contract_line' with mobile '_is_pack' method
Related card: https://trello.com/c/DM7aj133/1535-error-creaci%C3%B3-contracte-de-m%C3%B2bil-sense-sim
Contract creation by API accepts both contract_line
or contract_lines
because of an OTRS limitation (cannot send a list with just one element)