Unsetting field item properties via FieldItemBase::set() does not always work

Follow-up from #2021817-51: Make widgets / formatters work on EntityNG Field value objects.

+++ b/core/lib/Drupal/Core/Entity/Field/FieldItemBase.php
@@ -130,6 +130,7 @@ public function __unset($name) {
     $this->set($name, NULL);
+    unset($this->values[$name]);

This seems to be necessary, as otherwise there are bot fails, see #2014163-43: Bot checks for #2021817. So it looks like we've a problem in set().