BUG/MEDIUM: mux-spop: Truly drain outgoing data when the stream is closed

BUG/MEDIUM: mux-spop: Truly drain outgoing data when the stream is closed

After the H2 and the FCGI multiplexers, it is the third mux concerned by
this issue.

When we try to send data to the server and the stream is closed (in error,
in half-closed state or fully closed), remaining data must be drained. This
way the upper stream is able to properly handle the stream close.

However, there was a bug here. The mux claimed to have consumed these data
without draining them from the buffer. The issue was never reported on the
SPOP multiplexer. But, in theory, the same than for the FCGI multiplexer is
possible.

This patch must be backported as far as 3.2.