Fix serialization errors with Serde and add a serialization/deserialization test

[econaxis]

Right now, serializing items with Serde errors out because the types are wrong. For example, wheelchair boarding Availability serializes as i32 but deserializes as str. This leads to an error when we test round-trip serialization (serialize then deserialize).

I fixed and added one test for this issue.

[antoine-de]

do you need this to be released? if so you can increase the version in cargo.toml (the minor I think, since this can be a breaking change) and it will be released after the merge