Issue #3032674 by bsztreha, aprice42: Cache read problem in mailchimp_get_mergevars()

Cache setting bug where cache was not being stored with the correct key and was never retrievable.

mailchimp.module

@@ -289,7 +289,7 @@ function mailchimp_get_mergevars(array $list_ids, $reset = FALSE) {
           $mergevars[$list_id] = array_merge($mergevars[$list_id], $result->merge_fields);
         }
 
-        \Drupal::cache()->set($list_id . '-mergevars', $result->merge_fields);
+        $cache->set($list_id . '-mergevars', $mergevars[$list_id]);
       }
     }