Fix a frozen string error in app/mailers/notify.rb

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/mailers/notify.rb

@@ -166,7 +166,7 @@ class Notify < BaseMailer
     headers['In-Reply-To'] = message_id(model)
     headers['References'] = [message_id(model)]
 
-    headers[:subject]&.prepend('Re: ')
+    headers[:subject] = "Re: #{headers[:subject]}" if headers[:subject]
 
     mail_thread(model, headers)
   end

changelogs/unreleased/55104-frozenerror-can-t-modify-frozen-string.yml

+---
+title: Fix a frozen string error in app/mailers/notify.rb
+merge_request: 23683
+author:
+type: fixed

spec/mailers/notify_spec.rb

@@ -28,8 +28,8 @@ describe Notify do
   end
 
   def have_referable_subject(referable, reply: false)
-    prefix = referable.project ? "#{referable.project.name} | " : ''
-    prefix.prepend('Re: ') if reply
+    prefix = (referable.project ? "#{referable.project.name} | " : '').freeze
+    prefix = "Re: #{prefix}" if reply
 
     suffix = "#{referable.title} (#{referable.to_reference})"