add is_new check to #change.

Resource#create_model instantiates an activerecord model, but doesn't trigger a save. The save comes in _replace_fields, but if field_data[:attributes] is empty it never sets @save_needed.

I'm sure this isn't the best way to solve this issue, but it works for now.

Author: adamworrall

lib/jsonapi/resource.rb

@@ -46,7 +46,7 @@ def change(callback)
           @changing = true
           run_callbacks callback do
             yield
-            save if @save_needed
+            save if @save_needed || is_new?
           end
         end
       end