Httpun.Body.Writer.flush hangs on closed client connection (httpun-eio)

When

* using `httpun-eio`
* and `Httpun.Reqd.respond_with_streaming`
* and the client closes the connection prematurely

then

* the call to `Httpun.Body.Writer.flush` never calls the provided callback.

The expected behavior would be that `Httpun.Body.Writer.flush` calls its callback with the `Closed parameter.