code: def _set_default_headers(self):
          
          self.headers.setdefault('Date', self.get_current_time)
          self.headers.setdefault('Server', self.SERVER_INFO)
          self.headers.setdefault('Content-Length', "%d" % len(self.message))
          if self.app.enabled('x-powered-by'):
              self.headers.setdefault('X-Powered-By', 'Growler')
docs: Create some default headers that should be sent along with every HTTP
          response