Use the correct subject id for quoting

app/controllers/messages_controller.rb

@@ -110,7 +110,7 @@ class MessagesController < ApplicationController
     content = "#{ll(Setting.default_language, :text_user_wrote, user)}\\n> "
     content << text.to_s.strip.gsub(%r{<pre>((.|\s)*?)</pre>}m, '[...]').gsub('"', '\"').gsub(/(\r?\n|\r\n?)/, "\\n> ") + "\\n\\n"
     render(:update) { |page|
-      page << "$('reply_subject').value = \"#{subject}\";"
+      page << "$('message_subject').value = \"#{subject}\";"
       page.<< "$('message_content').value = \"#{content}\";"
       page.show 'reply'
       page << "Form.Element.focus('message_content');"