/erm/entitlements endpoint doesn't expand owner when entitlement is external

Overview:
Steps to Reproduce:

1. Create an agreement with an agreement line for a eHoldings resource
2. Retrieve agreement line using the /erm/entitlements endpoint
3. Look at "owner" object in returned JSON

Expected Results:
Owner would be expanded to include full details of the owning agreement

Actual Results:
Owner not expanded and only `id` and `name` included

Additional Information:
This impacts on the Orders app as they use the /erm/entitlements endpoint to retrieve and display information on Agreement Lines linked to a Purchase Order Line. The information they display includes the Agreement status - via the Owner object. However, this doesn't currently work for Agreement Lines/Entitlements which link to external resources (specifically eHoldings)

Interested parties:
dennisbridges
kgambrell
aliaksei_chumakou