fix check for temp record

Checking for the presence of the idField on a record is a better indicator of if the record is a temp.  It beats checking for the lack of an `__id` field, because in Vue it’s sometimes a pain in the butt to get rid of properties thanks to using getters and setters for reactivity. This won’t be necessary once 3.0 ships with proxy-based reactivity.

Author: marshallswain

src/service-module/service-module.mutations.ts

@@ -285,9 +285,9 @@ export default function makeServiceMutations() {
         : Model && _get(Model, `copiesById[${id}]`)
 
       if (copy) {
-        const original = copy.__isTemp
-          ? state.tempsById[id]
-          : state.keyedById[id]
+        const original = copy[state.idField]
+          ? state.keyedById[id]
+          : state.tempsById[id]
         mergeWithAccessors(copy, original)
       }
     },