ServerVector addFeaturesInternal fails when features lack an id

`ol.source.ServerVector::addFeatures` will only add the first of an array of features if the features lack an `id` property because `addFeaturesInternal` tries to track them by id to avoid loading the same feature twice.  But if the features return nothing for `getId()` then only the first feature will be loaded.
So if `featureId` is defined and is in `this.loadedFeatures_` already you're going to add to `notLoadedFeatures` and therefore pass it to the parent's `addFeaturesInternal`. Is this really what you want? I think it's only when `featureId` is `undefined` that you want to unconditionally add it to `notLoadedFeatures`, no?

```js
  if (!goog.isDef(featureId)) {
    notLoadedFeatures.push(feature);
  } else if (!(featureId in this.loadedFeatures_)) {
    notLoadedFeatures.push(feature);
    this.loadedFeatures_[featureId] = true;
  }
```