Creating a resource does not give an error when `linkage` isn't specified.

In the Peeps sample app the old format of specifying links was used:

curl -i -H "Accept: application/vnd.api+json" -H 'Content-Type:application/vnd.api+json' -X POST -d '{"data": {"type":"phone-numbers", "links": {"contact": {"type": "contacts", "id":"1"}}, "name":"home", "phone-number":"(603) 555-1212"}}' "http://localhost:3000/phone-numbers"

This does not raise an error and the phone number is created with out being associated to the contact. The correct format is:

curl -i -H "Accept: application/vnd.api+json" -H 'Content-Type:application/vnd.api+json' -X POST -d '{"data": {"type":"phone-numbers", "links": {"contact": {"linkage": {"type": "contacts", "id":"1"}}}, "name":"home", "phone-number":"(603) 555-1212"}}' "http://localhost:3000/phone-numbers"

We should raise an error when the format is not correct for an association.

Start with the Peeps sample app and reproduce the POST using the old link format, then compare it with the format that includes linkage. Trace the association request parsing and validation entry point, and add coverage showing that an association without linkage is rejected rather than silently creating an unassociated phone number.