HasOneNestedSchemaItem setValue: delegate to id schema item when setting association to another instance

When the parent resource sets the nested resource with a new instance of
the related type, and a subsequent fetch happens, changing the associated
nested resource id, some data from the old instance carries over to the
new instance.
Use getPath, and fixes getting the associated id schema item
Be more clear with expectations
Merge pull request #95 from zendesk/igor_josh/hasonenestedschemaitem_setvalue

HasOneNestedSchemaItem setValue: delegate to id schema item when setting association to another instance