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size(ContentLength). Closing DataStream does not close
RawStream. Options processed:
call(Closure, RawStream, BytesLeft) when DataStream is
closed. BytesLeft is the number of bytes of the range stream
that have not been read, i.e., 0 (zero) if all data has been
read from the stream when the range is closed. This was
introduced for supporting Keep-alive in http_open/3 to
reschedule the original stream for a new request if the data
of the previous request was processed.