Breaking Changes with the checkin endpoint

[HerrLevin]

Dear @Traewelling/api-consumer,

please note that we've increased the type safety in the endpoint /trains/checkin.
Properties like business or. visibility NEED to be of the numeric type and cannot be a number inside a string like "0". Booleans have to be true instead of "true", etc.

Since this broke a few apps we've temporarily reduced the type safety. This will be reverted in a few days so please fix your implementations if this applies to you.

Thank you. :)