[BUGFIX release] fix relationship merging

[stefanpenner]

When relationship information is merged, the newest information should win.
Unfortunately prior to this PR, relationship containing only link information, would mistakenly purge any existing data information on that relationship.

1. Update One

{
    data: { /* existing data */ },
    Link: { /* existing data */ },
}

2. Update Two

{
    Link: { /* new data */ },
}

Previously resulted in:

{
   link: { /* new data */ },
   data: null // oops
}

But now (correctly) results in:

{
   link: { /* new data */ },
   data: { /* existing data */ }
}

[hjdivad]

Fix looks great

Test looks good but could be minimized. Unless I'm missing something we can drop kid both in the modeling and the payload to make the test more focused.

[hjdivad]

Do we need this at all?

Can't we minimize the test to only have middle-ager and elder?

I think now that we've figured out the root cause the kid relationship adds noise to the test.