Use `headers` shorthand in mailers (#31956)

app/mailers/admin_mailer.rb

@@ -56,9 +56,11 @@ class AdminMailer < ApplicationMailer
   def new_critical_software_updates
     @software_updates = SoftwareUpdate.where(urgent: true).to_a.sort_by(&:gem_version)
 
-    headers['Priority'] = 'urgent'
+    headers(
-    headers['X-Priority'] = '1'
+      'Importance' => 'high',
-    headers['Importance'] = 'high'
+      'Priority' => 'urgent',
+      'X-Priority' => '1'
+    )
 
     locale_for_account(@me) do
       mail subject: default_i18n_subject(instance: @instance)

app/mailers/application_mailer.rb

@@ -16,8 +16,10 @@ class ApplicationMailer < ActionMailer::Base
   end
 
   def set_autoreply_headers!
-    headers['Precedence'] = 'list'
+    headers(
-    headers['X-Auto-Response-Suppress'] = 'All'
+      'Auto-Submitted' => 'auto-generated',
-    headers['Auto-Submitted'] = 'auto-generated'
+      'Precedence' => 'list',
+      'X-Auto-Response-Suppress' => 'All'
+    )
   end
 end

app/mailers/notification_mailer.rb

@@ -6,7 +6,10 @@ class NotificationMailer < ApplicationMailer
          :routing
 
   before_action :process_params
-  before_action :set_status, only: [:mention, :favourite, :reblog]
+  with_options only: %i(mention favourite reblog) do
+    before_action :set_status
+    after_action :thread_by_conversation!
+  end
   before_action :set_account, only: [:follow, :favourite, :reblog, :follow_request]
   after_action :set_list_headers!
 
@@ -18,7 +21,6 @@ class NotificationMailer < ApplicationMailer
     return unless @user.functional? && @status.present?
 
     locale_for_account(@me) do
-      thread_by_conversation(@status.conversation)
       mail subject: default_i18n_subject(name: @status.account.acct)
     end
   end
@@ -35,7 +37,6 @@ class NotificationMailer < ApplicationMailer
     return unless @user.functional? && @status.present?
 
     locale_for_account(@me) do
-      thread_by_conversation(@status.conversation)
       mail subject: default_i18n_subject(name: @account.acct)
     end
   end
@@ -44,7 +45,6 @@ class NotificationMailer < ApplicationMailer
     return unless @user.functional? && @status.present?
 
     locale_for_account(@me) do
-      thread_by_conversation(@status.conversation)
       mail subject: default_i18n_subject(name: @account.acct)
     end
   end
@@ -76,17 +76,21 @@ class NotificationMailer < ApplicationMailer
   end
 
   def set_list_headers!
-    headers['List-ID'] = "<#{@type}.#{@me.username}.#{Rails.configuration.x.local_domain}>"
+    headers(
-    headers['List-Unsubscribe'] = "<#{@unsubscribe_url}>"
+      'List-ID' => "<#{@type}.#{@me.username}.#{Rails.configuration.x.local_domain}>",
-    headers['List-Unsubscribe-Post'] = 'List-Unsubscribe=One-Click'
+      'List-Unsubscribe-Post' => 'List-Unsubscribe=One-Click',
+      'List-Unsubscribe' => "<#{@unsubscribe_url}>"
+    )
   end
 
-  def thread_by_conversation(conversation)
+  def thread_by_conversation!
-    return if conversation.nil?
+    return if @status.conversation.nil?
 
-    msg_id = "<conversation-#{conversation.id}.#{conversation.created_at.strftime('%Y-%m-%d')}@#{Rails.configuration.x.local_domain}>"
+    conversation_message_id = "<conversation-#{@status.conversation.id}.#{@status.conversation.created_at.to_date}@#{Rails.configuration.x.local_domain}>"
 
-    headers['In-Reply-To'] = msg_id
+    headers(
-    headers['References']  = msg_id
+      'In-Reply-To' => conversation_message_id,
+      'References' => conversation_message_id
+    )
   end
 end