Fix StreamExt::forward impl by calling poll_flush()

Author: ebkalderon

futures-util/src/stream/forward.rs

@@ -39,7 +39,7 @@ where
         Forward {
             sink: Some(sink),
             stream: stream.fuse(),
-                buffered_item: None,
+            buffered_item: None,
         }
     }
 
@@ -90,7 +90,7 @@ where
                 Poll::Ready(Some(Err(e))) => return Poll::Ready(Err(e)),
                 Poll::Ready(None) => {
                     try_ready!(self.as_mut().sink().as_pin_mut().expect(INVALID_POLL)
-                                   .poll_close(lw));
+                                   .poll_flush(lw));
                     return Poll::Ready(Ok(self.as_mut().sink().take().unwrap()))
                 }
                 Poll::Pending => {