Keep a code block's leading newline when linking a message's text

A <pre> whose text starts with a newline needs one more written out to
survive being parsed again, as the html5 serializer does with
preserve_newline. Without it, linking text dropped one more of a code
block's leading blank lines than rendering did before.

app/helpers/messages_helper.rb

@@ -130,7 +130,8 @@ module MessagesHelper
         end
       end
 
-      fragment.to_html.html_safe
+      # A <pre> starting with a newline needs an extra one written out to survive the next parse
+      fragment.to_html(preserve_newline: true).html_safe
     end
 
     def messages_actions