Fix readiness of buffered stream

If we see an empty slot then we're not immediately ready, but rather ready when
the underlying stream is ready, so propagate the readiness notification through
to that.

Closes #57

src/stream/buffered.rs

@@ -107,7 +107,8 @@ impl<S> Stream for Buffered<S>
         // If we've got an empty slot, then we're immediately ready to go.
         for slot in self.futures.iter() {
             if let State::Empty = *slot {
-                return task.notify()
+                self.stream.schedule(task);
+                break
             }
         }