docs: def start_response(self, status, headers, exc_info = None):
          """WSGI callable to begin the HTTP response."""
          # "The application may call start_response more than once,
          # if and only if the exc_info argument is provided."
          if self.started_response and not exc_info:
              raise AssertionError("WSGI start_response called a second "
                                   "time with no exc_info.")
          self.started_response = True
          
          # "if exc_info is provided, and the HTTP headers have already been
          # sent, start_response must raise an error, and should raise the
          # exc_info tuple."
          if self.req.sent_headers:
              try:
                  raise exc_info[0], exc_info[1], exc_info[2]
              finally:
                  exc_info = None
          
          self.req.status = status
          for k, v in headers:
              if not isinstance(k, str):
                  raise TypeError("WSGI response header key %r is not a byte string." % k)
              if not isinstance(v, str):
                  raise TypeError("WSGI response header value %r is not a byte string." % v)
          self.req.outheaders.extend(headers)
          
          return self.write