FIX: Don't diplay character reference in HTML diffs (#4204)

* FIX: Don't diplay character reference in HTML diffs

Before this change, HTML escaping was done before splitting text into
tokens, so token splitter saw literals like "'", and split them as
it was normal text into parts into ["&", "#", "39", ";"]. This caused
diff to display character references, as those tokens used separate
HTML tags to display their insertion/deletion status.

* Avoid making one element arrays while generating diffs

lib/discourse_diff.rb

@@ -261,8 +261,7 @@ class DiscourseDiff
     end
 
     def characters(string)
-      string = CGI::escapeHTML(string)
-      @tokens.concat string.scan(/(\W|\w+[ \t]*)/).flatten
+      @tokens.concat string.scan(/\W|\w+[ \t]*/).map { |x| CGI::escapeHTML(x) }
     end
 
   end