Encode Page-Title header as ISO-8859-1

app/controllers/application_controller.rb

@@ -339,6 +339,7 @@ class ApplicationController < ActionController::Base
   end
 
   def set_page_title_header
-    response.headers['Page-Title'] = page_title('GitLab')
+    # Per https://tools.ietf.org/html/rfc5987, headers need to be ISO-8859-1, not UTF-8
+    response.headers['Page-Title'] = page_title('GitLab').encode('ISO-8859-1')
   end
 end