Prevent pending_open streams from being released. (#295)

* Prevent `pending_open` streams from being released.

This fixes a panic that would otherwise occur in some cases. A test
demonstrating said panic is included.

* Clear the pending_open queue together with everything else.

src/proto/streams/send.rs

@@ -417,6 +417,7 @@ impl Send {
     pub fn clear_queues(&mut self, store: &mut Store, counts: &mut Counts) {
         self.prioritize.clear_pending_capacity(store, counts);
         self.prioritize.clear_pending_send(store, counts);
+        self.prioritize.clear_pending_open(store, counts);
     }
 
     pub fn ensure_not_idle(&self, id: StreamId) -> Result<(), Reason> {