Return resource version in the header

When one does ``api/articles`` they should know which resource's version served their response.

So we can return the version for all cases (e.g. also in ``api/v1.0/articles``)
Actually this is intentional.

The `$response['headers']` only contains the contents of the header, so in this case if we want to detect `Vary: X-API-Version` we need to find `'X-API-Version'`. On the other hand if we want to detect `X-API-Version: v1.1` we'll look for `'v1.1'`.