fixed handling checkbox custom option

[gibkigonzo]

Related issues

closes #2781

Short description and why it's useful

In checkbox there can be multiple selections so its value should be send as string of concatenated ids, eg: '1,2,3'

Which environment this relates to

Check your case. In case of any doubts please read about Release Cycle

• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility and no breaking changes)
• I've updated the Changelog on how to port existing VS sites with this new feature

IMPORTANT NOTICE - Remember to update CHANGELOG.md with description of your change

Contribution and currently important rules acceptance

• I read and followed contribution rules

[pkarw]

Great job 👍

[andrzejewsky]

@gibkigonzo Please move functions that you have created (and the old ones) to some helpers

[andrzejewsky]

looks good now 👍 good job!