Don't bypass stream fuse in 0.1 Forward

The poll implementation for Forward used method stream_mut
to get at the stream. This bypasses the Fuse that was meant to
prevent stream from being polled more than once in case
close() on the sink initially returns NotReady.

src/stream/forward.rs

@@ -91,7 +91,7 @@ impl<T, U> Future for Forward<T, U>
         }
 
         loop {
-            match self.stream_mut()
+            match self.stream.as_mut()
                 .expect("Attempted to poll Forward after completion")
                 .poll()?
             {