Use merge instead of dup

ee/lib/gitlab/elastic/indexer.rb

@@ -106,9 +106,9 @@ module Gitlab
       end
 
       def elasticsearch_config(target)
-        config = Gitlab::CurrentSettings.elasticsearch_config.dup
-        config[:index_name] = target.index_name
-        config.to_json
+        Gitlab::CurrentSettings.elasticsearch_config.merge(
+          index_name: target.index_name
+        ).to_json
       end
 
       def gitaly_connection_info

ee/spec/lib/gitlab/elastic/indexer_spec.rb

@@ -282,8 +282,7 @@ describe Gitlab::Elastic::Indexer do
   end
 
   def elasticsearch_config
-    config = Gitlab::CurrentSettings.elasticsearch_config.dup
-    config.merge!(
+    Gitlab::CurrentSettings.elasticsearch_config.merge(
       index_name: 'gitlab-test'
     )
   end