{backport release} [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 */ }
}

[stefanpenner]

I think this may due to the serializer change between release and beta, will investigate.