Issue #2353695 by ultimike: D6->D8 Migration missing variable: feed_item_length.

core/modules/migrate_drupal/config/install/migrate.migration.d6_system_rss.yml

@@ -6,8 +6,10 @@ source:
   plugin: variable
   variables:
     - feed_default_items
+    - feed_item_length
 process:
   'items/limit': feed_default_items
+  'items/view_mode': feed_item_length
 destination:
   plugin: config
   config_name: system.rss

core/modules/migrate_drupal/src/Tests/Dump/Drupal6SystemRss.php

@@ -25,6 +25,10 @@ public function load() {
       'name' => 'feed_default_items',
       'value' => 'i:10;',
     ))
+    ->values(array(
+      'name' => 'feed_item_length',
+      'value' => 's:5:"title";',
+    ))
     ->execute();
   }
 }

core/modules/migrate_drupal/src/Tests/d6/MigrateSystemRssTest.php

@@ -38,6 +38,7 @@ protected function setUp() {
   public function testSystemRss() {
     $config = \Drupal::config('system.rss');
     $this->assertIdentical($config->get('items.limit'), 10);
+    $this->assertIdentical($config->get('items.view_mode'), 'title');
   }
 
 }