Backport modelName fix from #5406 to 3.1 (#5601)

* [BUGFIX release] Fix `Model.modelName` inheritance with Ember 3.2+.

On Ember 3.2 and higher, Ember uses "real" inheritance when an
`Ember.Object` is `.extend()`ed. This means that in 3.2 and higher the
`ModelClassHere.modelName` property is now being inherited (where
previously each `.extend()` reset the value to `null`).

The fix here ensures that each `ModelClass.extend()` gets its own
`modelName`...

* [BUGFIX release] Fix toString test to work with Ember 3.2+.

This test was attempting to ensure that the records guid was included in
the `record.toString()` output, but changes in Ember 3.2+ changed the
default output of the class / constructor name portion (therefore
failing the assertion in a useless way).

This updates the test to add a custom `toString` to the model class so
that we can be decoupled from how Ember `.toString()`'s classes when
there is no custom `.toString` setup (because this is actually fairly
unstable version over version).

* fix branch

addon/-private/system/store.js

@@ -2132,7 +2132,10 @@ Store = Service.extend({
       assert(`'${inspect(klass)}' does not appear to be an ember-data model`, klass.isModel);
 
       // TODO: deprecate this
-      klass.modelName = klass.modelName || modelName;
+      let hasOwnModelNameSet = klass.modelName && klass.hasOwnProperty('modelName');
+      if (!hasOwnModelNameSet) {
+        klass.modelName = modelName;
+      }
 
       this._modelFactoryCache[modelName] = factory;
     }

tests/unit/model-test.js

@@ -19,6 +19,7 @@ module('unit/model - DS.Model', {
       name: DS.attr('string'),
       isDrugAddict: DS.attr('boolean')
     });
+    Person.toString =  () => 'person';
 
     env = setupStore({
       person: Person
@@ -269,7 +270,7 @@ test("a record's id is included in its toString representation", function(assert
     });
 
     return store.findRecord('person', 1).then(record => {
-      assert.equal(record.toString(), `<(subclass of DS.Model):${guidFor(record)}:1>`, 'reports id in toString');
+      assert.equal(record.toString(), `<person:${guidFor(record)}:1>`, 'reports id in toString');
     });
   });
 });