Prevent slot not found error when server has no servlets

When a freshly cloned webserver has no servlets a 'keys' not found slot error occurs when an url is retrieved. This is because the 'servlets' field is 'nil' rather than a dictionary.

The fix here was to initialize the slot to a new dictionary. I also had to change the result returned when the servlet is not found so that it is an response object rather than a string to avoid other slot not found errors.