Don't overwrite headers that were already set (#14699).

git-svn-id: http://svn.redmine.org/redmine/trunk@13784 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/mailer.rb

@@ -370,7 +370,7 @@ class Mailer < ActionMailer::Base
   end
 
   def mail(headers={}, &block)
-    headers.merge! 'X-Mailer' => 'Redmine',
+    headers.reverse_merge! 'X-Mailer' => 'Redmine',
             'X-Redmine-Host' => Setting.host_name,
             'X-Redmine-Site' => Setting.app_title,
             'X-Auto-Response-Suppress' => 'OOF',