Ensure forward compatibility of Items with unknown content types

[Dav1dde]

Now since the serialization format and schema may change in the future, we need to stay forward compatible. A customer should not have to upgrade an older Relay when we change the format (e.g. json to something else).

We need to know not only forward unknown items, but also known items with an unknown content type. We can narrow the content type down to application/vnd.sentry.* and all other content types we can assume the 'default'.