You're running into an issue which has been there for years with DNN... Here's the very simple fix - make sure you save the file with a UTF-8 Encoding. More than likely, you're saving as ANSI which is causing the stripping of the "+" signs... If you're using Notepad, use the save as option and then change the encoding to "UTF-8".
Please let me know if this doesn't resolve your issue.
Thanks
Sanjay