Fixed banner.nse to remove surrounding whitespace in the banner.

For example, this removed SSH greetings final two characters: carriage return
and newline.

scripts/banner.nse

@@ -61,7 +61,7 @@ function grab_banner(host, port)
     return nil
   end
 
-  return response
+  return response:match("^%s*(.-)%s*$");
 
 end