Missing bookshelf link means loans may not persist on client side

The OPDS spec doesn't require a link with `rel="http://opds-spec.org/shelf"`, but without that link (and it looks like bookserver doesn't provide one) there's no way for a client to get a canonical list of a patron's currently active loans.

In practical terms, this means that the SimplyE mobile app loses all of a patron's loans when it's restarted. It looks for a bookshelf link, the link isn't there, so SimplyE assumes that all the loans have expired.

SimplyE could probably do a better job here, but when you're serving time-limited loans, the server is the source of truth about the loans.

No file or test is named; start by locating the API server's OPDS response for a patron's active loans. Check how the `http://opds-spec.org/shelf` link should identify the canonical active-loan list, then verify the response supplies it so SimplyE can restore loans after restart.

Written by the indexing model from the issue text.