Enqueue RST_STREAM, otherwise we can get stuck due to nothing enqueued to be sent. Then, `h2o_http2_stream_proceed` that closes the stream will never be called from `on_write_callback`.
Enqueue RST_STREAM, otherwise we can get stuck due to nothing enqueued to be sent. Then, `h2o_http2_stream_proceed` that closes the stream will never be called from `on_write_callback`.

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