BUG/MINOR: h2: report asynchronous end of stream on closed connections

BUG/MINOR: h2: report asynchronous end of stream on closed connections

Christopher noticed that the CS_FL_EOS to CS_FL_REOS conversion was
incomplete : when the connectionis closed, we mark the streams with EOS
instead of REOS, causing the loss of any possibly pending data. At the
moment it's not an issue since H2 is used only with a client, but with
servers it could be a real problem if servers close the connection right
after sending their response.

This patch should be backported to 1.8.