Fix poll_flush after poll_shutdown (#836)

Author: bdbai

src/codec/framed_write.rs

@@ -23,6 +23,7 @@ macro_rules! limited_write_buf {
 pub struct FramedWrite<T, B> {
     /// Upstream `AsyncWrite`
     inner: T,
+    final_flush_done: bool,
 
     encoder: Encoder<B>,
 }
@@ -89,6 +90,7 @@ where
         };
         FramedWrite {
             inner,
+            final_flush_done: false,
             encoder: Encoder {
                 hpack: hpack::Encoder::default(),
                 buf: Cursor::new(BytesMut::with_capacity(DEFAULT_BUFFER_CAPACITY)),
@@ -165,7 +167,10 @@ where
 
     /// Close the codec
     pub fn shutdown(&mut self, cx: &mut Context) -> Poll<io::Result<()>> {
-        ready!(self.flush(cx))?;
+        if !self.final_flush_done {
+            ready!(self.flush(cx))?;
+            self.final_flush_done = true;
+        }
         Pin::new(&mut self.inner).poll_shutdown(cx)
     }
 }