Nginx proxy_pass weird url escaping? (try it)

Source URL:

http://www.sitmo.com/gg/latex/latex2png.2.php?z=100&eq=%5Ccos%20%7Bx%7D%2B%5Csin%20%7Bx%7D%3DCV

For some sandbox Flash security reasons I need to create a proxy on my domain,

   location /latex_gen/ {
       proxy_pass    http://www.sitmo.com/gg/latex/latex2png.2.php?z=100&eq=;
   }

      

        
        
        
      

    

Which now points to http://www.edoboard.com/latex_gen/%5Ccos%20%7Bx%7D%2B%5Csin%20%7Bx%7D%3DCV

If you look at these two urls, they generate images, but in the second case the "+" is missing. Is it due to some kind of acceleration?