fixup! Re-design the StreamMuxer trait

First send Close frame, then flush the stream. Doing things in this
order makes the tests pass without knowing exactly how much data
we need to read.

muxers/mplex/src/lib.rs

@@ -207,8 +207,8 @@ where
         let this = self.get_mut();
         let mut io = this.io.lock();
 
-        ready!(io.poll_flush_stream(cx, this.id))?;
         ready!(io.poll_close_stream(cx, this.id))?;
+        ready!(io.poll_flush_stream(cx, this.id))?;
 
         Poll::Ready(Ok(()))
     }