Break js-data relations when JsonApi related data changes or becomes Null or Empty

[BlairJ]

js-data maintains data relationships.

If however other changes are being made against the server and we then request a resource that subsequently returns relationship data that differs from the js-data data store, we should try to update the datastore to keep it in sync?

Should we try to do this or leave to the developer to keep things in sync ??
See also #9

Cases:

• We request a to one related field and it returns null (from the server).
  • Clear the local key on the parent object
• We request a toMany relationship that returns an empty array
  • Clear all foreign keys on foreign objects

[BlairJ]

Related to #42

[J Rob Gant]

I definitely think that the server is the source of truth and so if we get differing information back from the server then js-data should respect that.

I've recently had to add onConflict: 'replace' to my DS#save and DS#find calls so that it doesn't try to merge nested objects an create problems for my app.