code: def refresh():
      
      old_token = guard.read_token_from_header()
      new_token = guard.refresh_jwt_token(old_token)
      ret = {'access_token': new_token}
      return flask.jsonify(ret), 200
docs: Refreshes an existing JWT by creating a new one that is a copy of the old
      except that it has a refrehsed access expiration.
  
      .. example::
         $ curl http://localhost:5000/refresh -X GET \
           -H "Authorization: Bearer <your_token>"