Finishes this response, ending the HTTP request.

def finish(self):
        """Finishes this response, ending the HTTP request."""
        if self._finished:
            raise RuntimeError("finish() called twice.  May be caused "
                               "by using async operations without the "
                               "@asynchronous decorator.")
        if not hasattr(self, '_stream_queue') or not self._stream_queue:
            raise RuntimeError("`_stream_queue` was not assigned, you should"
                               "call `write_stream_queue` to set.")

        # === Replace `if not self._headers_written` === #
        self.set_status(200)
        self.set_header("Content-Type", "text/event-stream")
        self.set_header("Cache-Control", "no-cache")
        self.set_header("Access-Control-Allow-Origin", "*")
        # ============================================== #

        self.request.connection.set_close_callback(None)

        # === Add before self.flush === #
        # Reset buffer
        self._write_buffer = []
        self._headers_written = False
        # ============================= #

        self.flush(include_footers=True)

        # === Add after self.flush === #
        self._write_buffer = self._stream_queue
        self.request.connection._write_buffer = self._stream_queue
        # ============================ #

        self.request.finish()
        self._log()
        self._finished = True
        self.on_finish()
        # Break up a reference cycle between this handler and the
        # _ui_module closures to allow for faster GC on CPython.
        self.ui = None