Ignore NotConnected error in poll_shutdown()

We try to write buffered data to the stream before shutting it down. This
seems to be in line with the guidance from Tokio's AsyncWrite docs:

> Invocation of a shutdown implies an invocation of flush. Once this method
> returns `Ready` it implies that a flush successfully happened before the
> shutdown happened. That is, callers don't need to call flush before
> calling shutdown. They can rely that by calling shutdown any pending
> buffered data will be written out.

We propagate errors from the write. However, if we get a `NotConnected`
error during shutdown we might just as well ignore it.

src/common/mod.rs

@@ -333,7 +333,11 @@ where
 
     fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
         while self.session.wants_write() {
-            ready!(self.write_io(cx))?;
+            match ready!(self.write_io(cx)) {
+                Ok(_) => {}
+                Err(err) if err.kind() == io::ErrorKind::NotConnected => {}
+                Err(err) => return Poll::Ready(Err(err)),
+            }
         }
         Pin::new(&mut self.io).poll_shutdown(cx)
     }